fix: a readout keeps its metric prefix when the stream declares no unit

The SI prefix ladder in format_measure() was gated on a non-empty unit, so two
decimals of a small unitless value printed `0.00` -- the reading gone, not merely
coarse. That is a whole class of channels rather than a corner: a CONFIG need not
declare a unit, and a math channel never has one. Below 1e-12 the same loss
happened even with a unit, so that range falls back to three significant digits.

With no unit the prefix goes tight against the number (`585.24m`), since the space
exists to separate the number from its unit and a spaced `m` alone reads as metres.

The mock declared `unit="V"` for every data signal, which is why no probe could see
this; it now takes `--unit ''`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -449,13 +449,28 @@ Two lines of the self-check output are assertions in disguise:
   equal `origin + first_rel` as a *tick label* plus its caption, i.e. the labels and the
   caption must share one anchor. `x_origin_set` is read back out of the plot; it must be 0 in
   the absolute form.
-- `[fit-probe] <action> window=.. x=[..] span=.. y=[..]` (stderr, `UDPSCOPE_PROBE_FIT_MS`) —
-  the two fits, through the same `RowSet` calls the right-click menu wires. It first puts
-  the y scale and the window where the data is *not* (a fixed `[-100, 100]`, a 50 ms
-  window), so the assertions have something to be wrong about: **`fit x` must widen the
-  window to the data's span and leave `y` exactly as it was**, and **`fit all` must then
-  move `y` to the data's own range**. A `y` that changed on the `fit x` line is the two
-  paths aliased.
+- `[fit-probe] <action> pinned=D window=.. event_span=.. x=[..] span=.. y=[..]` (stderr,
+  `UDPSCOPE_PROBE_FIT_MS`) — the two fits, through the **menu handlers** rather than the bare
+  `RowSet` calls, because the notice and the control are the handler's half. A fit is the **undo
+  of an x zoom**, so the probe performs one: it moves the y scale off the data (a fixed
+  `[-100, 100]`), sets a 2 s window through the control's own path, then zooms row 0 to a quarter
+  of it the way a gesture does — on the *member* axis — and fits. Four assertions.
+  **`window` and `event_span` must be unchanged on every line** and equal to the typed 2 s: a fit
+  is a plot interaction and may not restate the time scale (see the invariant below), and they are
+  the only two readings that can see it try. The `zoomed` line's `span` must be a quarter of the
+  window, or the fits have nothing to undo and the lines below mean nothing. **`fit x` must put
+  `span` back to the whole window and leave `y` exactly as it was**, and **`fit all` must do the
+  same and move `y` to the data's own range** — a `y` that changed on the `fit x` line is the two
+  paths aliased. `x`'s *position* advances between lines on a live view and is not an assertion:
+  the follow is re-armed on the newest data, which is what the whole window means there.
+  Run it **inside a pause** as well (`UDPSCOPE_PROBE_PAUSE_MS=2000 UDPSCOPE_PROBE_FIT_MS=2200`),
+  which is the other branch and the one that can silently revert: `pinned=1`, and `fit x` must
+  restore the *pin's* full range — the snapshot's own stamps, legitimately narrower than the
+  requested window (`span=1.179900` against `window=2.000000` is correct, not a finding) — and
+  must still read `pinned=1` afterwards. The pin is restored by writing the **members** as well as
+  the group, because `hold_pinned_window()` prefers a member that has moved since `pinned_stated_`
+  — that is how it tells a gesture from its own echo — so a fit that wrote the group alone reads
+  back as the zoomed range one frame later.
 - `[layout-probe] <action> rows=N buttons=B done=D done_box=[x,y,WxH] rows_y=[..] close=[..] add=[..] traces=[..]`
   (stderr, `UDPSCOPE_PROBE_LAYOUT_MS`) — **`buttons` must be `2N+1`** while the editor is
   up (one `×` per row, one `+` per boundary) and **0 on the `editor off` line**: the
@@ -749,6 +764,47 @@ far the largest file.
   again until the next latch. `PlotArea::create()` is handed `App::placeholder_plot`,
   never a row: the area keeps a reference to the plot it was created with, and a rebuild
   destroys every row.
+- **A readout quotes a stored sample, never a value between two of them.** The hover
+  crosshair and the cursors pane's `mix` values are one call, `Plot::crosshair_value()`, and it
+  reports the **nearer** of the two samples bracketing the x (a tie reads the earlier). Upstream
+  interpolated linearly, and that is wrong twice over here: ingest decimation is a *boxcar
+  average* over `time_stride` samples, so a number between two stored samples is a second
+  estimate laid over the first that nothing on screen or in an export holds; and past the sample
+  spacing the polyline is a line the renderer drew, so a point on it reports the drawing. The
+  change is in `deps/plotter`, which keeps the blend as `CrosshairValue::Interpolated` for a host
+  whose x is a quantity it is willing to interpolate — the nearest sample is the **default** and
+  **udpscope never states the other**, so a `set_crosshair_value_mode()` call appearing here is a
+  regression, not a feature.
+  **Both the mark and the number come from one call, `Plot::crosshair_point()`**, which returns
+  the quoted sample's own **x** as well as its y — so every mark this app draws for a readout is
+  *snapped to the sample it quotes*. The library's `CrosshairRow::marker` and the cursors pane's
+  `mix` crossing annotations (`place_crossings()`, which must use `crosshair_point()` and not
+  `crosshair_value()`) are both placed from it. Two consequences that look like defects and are
+  not: the mark **sits beside the polyline at a deep zoom** (one y feeding both the number and the
+  dot is what keeps them from disagreeing), and a crossing dot **sits a little off its own
+  vertical cursor line** — up to half a sample period, invisible at a whole-window view and the
+  width of the panel at a few samples across, which is the only zoom at which anyone is asking
+  which point it is. The line says where the *marker* is; the dot says which *datum* is quoted.
+  The hover mark is also deliberately *larger* than a trace's own sample dots
+  (`Style::crosshair_marker_px` 9 against `marker_px` 3), because a snapped mark lands among them
+  and at their size reads as one more sample. The guards are the library's
+  `crosshair_value_is_the_nearest_sample`, `the_crosshair_marker_snaps_to_the_sample_it_reports`,
+  `crosshair_interpolated_mode_blends_the_two_samples` and
+  `the_crosshair_mark_is_bigger_than_a_sample_dot`, plus `[cursors-probe]`'s `first mark at x=..
+  y=..` agreeing with the pane's first value.
+- **`format_measure()` is the one formatter for both panes, and its SI prefix does not depend on
+  there being a unit.** The cursors pane's values and the measurements pane's `text=` go through
+  it, so a change reaches both. The prefix ladder used to be inside `if (!unit.empty())`, which
+  made two decimals of a small unitless value read `0.00` — the reading gone, not merely coarse —
+  and that is a whole class of channels, not a corner: `measure_unit()` returns `""` for any signal
+  whose CONFIG declares no unit, and a **math channel never has one**. The regression net could not
+  see it because `tools/mock_udpstreamer.py` declared `unit="V"` for every data signal; it now takes
+  **`--unit ''`**, and that is the flag to run the measures and cursors probes under when touching
+  this (`text=[636.62m]`, `● sine=585.24m`). With no unit the prefix is written **tight** against
+  the number — `1.34m`, not `1.34 m` — because the space exists to separate the number from its
+  unit and a spaced `m` alone reads as metres; with one it stays spaced and the prefix joins the
+  unit (`636.62 mV`). Below `1e-12` the ladder runs out, and `%.2f` would print `0.00` again, so
+  that range falls back to `%.3g`.
 - **The cursors are read back from the plots only while a gesture is on that row.** `Cursor::x`
   is the *free* pointer position while a drag runs and the snapped value otherwise -- which is
   what the app itself wrote a frame earlier, or the snap of it. A tick that reads a row on
@@ -1328,12 +1384,37 @@ far the largest file.
   that grew from `1 s` to `123.456 ms` would widen the bar and relayout everything under it
   on a mere change of window. That is also why the window is typed into a popover rather
   than shown in the bar.
-- **`apply_time_window()` is the only place the display window changes.** It clamps to
-  `window_limit()` and moves `RowSet::set_time_window()` and `ScopeModel::set_event_span()`
-  together, because the trigger's pre-trigger share is a share of *this* window. Paths that
-  change the span for their own reasons (Fit, an event pin) call `sync_window_display()`
-  instead, which only restates the control and never moves the plot — calling `apply_` there
-  would clamp a legitimately short capture up to the requested window.
+- **No plot interaction changes the time scale; only the window control does.** The *time scale*
+  is more than the axis: `RowSet::window_s_` also feeds `ScopeModel::set_event_span()` (the
+  trigger's pre-trigger share is a share of *this* window), `ScopeModel::set_target_span()`
+  (which may re-plan the partition, move the decimation and **drop the history**), the window
+  control's readout and the session file. A **zoom or a pan** does not touch it by construction —
+  the controller writes the *member* axis and `clamp_to_data()` caps the span at `window_s_`, so
+  nothing has to be written to keep it that way. **Neither do the two fits**: a fit is the
+  *undo of a zoom*, so `RowSet::fit_x()` restores the window that is already set and reads
+  `window_s_` without ever writing it. Both fits used to frame the *data* instead and move
+  `window_s_` alone, which was wrong twice over — a gesture-triggered re-plan of the rings, and
+  an event span left on the *old* window until the next decade step or unit change applied the
+  fitted value for real (one action, two changes of time scale).
+- **`apply_time_window()` is the only place the time scale changes**, and the window control is
+  its only caller. It clamps to `window_limit()` and moves `set_target_span()`,
+  `set_event_span()`, `RowSet::set_time_window()`, the control and the session save together —
+  the partition **before** the axis, so the axis is framed over the rings that will exist. A fit
+  must not come here for two reasons: it is not entitled to the scale at all, and
+  `set_time_window()` unpins and re-arms the live follow, which would throw a held event or a
+  paused view onto a live edge it has nothing bound to. Paths that change the span for their own
+  reasons (an event pin) call `sync_window_display()` alone, which only restates the control and
+  never moves the plot — calling `apply_time_window()` there would clamp a legitimately short
+  capture up to the requested window.
+- **`fit_x()` restores the *whole of what the view may show*, which is not always `window_s_`.**
+  Live, that is the newest `window_s_` at the live edge (`set_time_window(window_s_)`, the same
+  path a resume takes). Pinned — a held event, a filling capture or a paused view — it is the
+  **pin's** full range, which is the snapshot's own stamps and may be legitimately *narrower*
+  than the requested window. It must write the per-row **member** axes as well as the link group:
+  `hold_pinned_window()` prefers a member that differs from `pinned_stated_`, which is how it
+  tells a real gesture from its own echo, so writing the group alone is reverted to the zoomed
+  range on the next frame. `fit_all()` is the y fit plus this, shared rather than duplicated so
+  the two cannot drift apart.
 - **The header bar's buttons are icon-only**, so headless probes must reach them
   through the `App` pointers (`trigger_toggle`, `pause_button`, …), not via
   `find_labelled_button()`. The **pane switches are toolbar buttons** too (`cursors_toggle`,

README.md

@@ -983,9 +983,9 @@ that changes nothing still says why. Each plot's y axis has a right-click menu
 with auto scale, a fixed range, **Fit X axis**, **Fit all**, and one
 **Remove '<signal>' from plot N** entry per occupied trace slot (a plot with nothing on
 it says so instead). The two fits are the same pair the overflow menu and the `f` key
-offer, split by what they touch: **Fit X axis** frames the window on the data the ring
-holds and leaves every y scale exactly as the user set it, **Fit all** does that and
-auto-scales the y axes too.
+offer, split by what they touch: **Fit X axis** puts the x view back on the whole time
+window and leaves every y scale exactly as the user set it, **Fit all** does that and
+auto-scales the y axes too. Neither changes the time window itself — see below.
 
 **The plot does not twitch.** Two independent causes were fixed, both of which
 showed up as the x axis moving a few pixels every frame — including while paused,
@@ -1139,10 +1139,39 @@ is what made a dragged cursor spring to where it had been. A ruler is per row: e
 two different values, and the pane names the row.
 
 `mix` needs no rulers: it reports the value the trace actually has at each marker, read
-through the library's `Plot::crosshair_value()` — the two bracketing samples,
-interpolated — so it is the answer to "what is this signal at these two times" rather
-than "drag a ruler until it crosses the trace". A readout is `--` when the trace has
-nothing at that time and `partial` when only one of the two markers is over samples.
+through the library's `Plot::crosshair_value()` — **the nearer of the two bracketing
+samples**, never a number between them — so it is the answer to "which datum of this
+signal is at these two times" rather than "drag a ruler until it crosses the trace". A
+readout is `--` when the trace has nothing at that time and `partial` when only one of
+the two markers is over samples.
+
+The value used to be a linear interpolation between the brackets, which is upstream's
+old convention and was wrong for this app in two ways at once. The samples here are
+**already averaged** — ingest decimation is a boxcar over `time_stride` samples (see
+"Decimation at ingest is an average") — so a number computed *between* two of them is a
+second, undeclared estimate laid over the first, and nothing on screen or in an export
+holds it. And at a zoom past the sample spacing it is visibly not the datum the pointer
+is over: the polyline between two samples is a straight line the renderer drew, and
+quoting a point on it reports the drawing rather than the data. A tie reads the earlier
+sample. The fix is upstream (`deps/plotter`, `crosshair_span()`'s `t` is a selector rather
+than a blend factor), so the library's own hover crosshair changed with the pane — which is
+the point, since both are the same lookup and a user comparing them must not see two
+numbers. Upstream keeps the blend as `CrosshairValue::Interpolated` for hosts whose x is a
+quantity they are willing to interpolate; the nearest sample is the default and udpscope
+never states the other.
+
+**The crossing dot is on the sample, not on the marker's column.** The dot is placed
+through `Plot::crosshair_point()` — the same lookup as the value, returning the quoted
+sample's own x as well as its y — so the mark the eye follows and the number in the pane
+are the same datum. The two x's differ by up to half a sample period: nothing at a
+whole-window view, and the whole width of the panel once a few samples span the axes, which
+is exactly the state in which a reader is asking "which point is that?". It therefore leaves
+the dot a little off its own vertical line by design — the line says where the *marker* is
+and the dot says which *datum* is being quoted, and when the marker is between two samples
+those are two places. The library's hover crosshair snaps the same way and draws its mark
+`Style::crosshair_marker_px` across (9 logical px against a trace's own 3 px sample dots),
+since a snapped mark lands among those dots and at their size would read as one more sample
+instead of as the answer to the question the hover asks.
 
 The readouts are a **table**: a header naming the columns, then one row per instrument —
 `| who | A | B | delta |`. The first row is the two times and their difference, and the
@@ -1156,7 +1185,16 @@ Every readout is a **measurement**, not a raw number: at most two decimals, scal
 SI prefix so the digits sit in `1..999`, and the unit the stream declared appended —
 `10.52 mV`, `-9.8 V`, `dT -398.67 ms`, `1.06 pV`. Two decimals is the resolution the
 instruments can claim (a marker lands on a sample, and the sample is an `f32`), and a
-trailing zero is dropped, so `1.50 V` reads `1.5 V`. A row's rulers use its signals' unit
+trailing zero is dropped, so `1.50 V` reads `1.5 V`. The prefix is applied **whether or not
+the stream declared a unit**, and that is not a detail: a signal whose CONFIG declares none is
+ordinary, a *math* channel never has one, and two decimals of an unprefixed small value is
+`0.00` — the reading was simply gone for an entire class of channels, which went unnoticed
+because the mock declares `V` for everything (it now takes `--unit ''`, which is the only way
+a probe can see this). With no unit the prefix is written *tight* against the number —
+`585.24m`, not `585.24 m` — because the space is there to separate the number from its unit,
+and a spaced `m` with nothing after it reads as metres. Below `1e-12`, where no prefix can
+help, the value falls back to three significant digits in whatever form fits rather than to
+`0.00`. A row's rulers use its signals' unit
 when they agree on one and print bare digits when they do not: a ruler is on the row's y
 axis, and two units on one axis make "the" unit a guess. A trace's line carries the
 signal's **colour chip and its name** — `● sine  616.49 mV  -24.57 mV  -641.06 mV` — and the
@@ -1247,10 +1285,24 @@ logged with both rates and the new anti-alias corner. A window change that leave
 limitations below); only the shared clock is decimated.
 
 **Fit to data** (overflow menu, the `f` key, or **Fit all** in a plot's right-click menu)
-auto-scales every row's y axis and re-frames the x window on the data the ring holds,
-propagated to every row. **Fit X axis** is the same re-framing without the y scaling, for
-the common case where the window should be the data's but the levels were placed by hand.
-The status line acknowledges either (`x fitted to the buffered data (1.25 s)`).
+auto-scales every row's y axis to the data it holds and puts the x view back on the whole time
+window, propagated to every row. **Fit X axis** is that x half alone, for the common case where
+a zoom should be undone but the levels were placed by hand. The status line acknowledges either
+(`x back to the 2 s window`).
+
+**Nothing you do on the plot changes the time window** — only the window control does. A fit is
+the *undo of a zoom*: a 2 s window zoomed into reads 2 s again. That matters because the window
+is not just the axis. It is also the trigger's event span, how the samples are stored (a window
+long enough to move the decimation re-plans the rings and **drops the history**), the control's
+readout and what the session file records — so a wheel notch or a Fit that redefined it would
+quietly throw the buffer away. A zoom and a pan are view-only by construction; the fits read the
+window and never write it.
+
+Which 2 s depends on the state, and the fit restores the whole of whatever the view is allowed to
+show: live, the newest window at the live edge; and for a held event or a paused view, the pin's
+full range — the snapshot's own span, which is legitimately *narrower* than the requested window
+when the ring had not filled it. A fit of a frozen view keeps it frozen rather than throwing it
+back onto the stream.
 
 **Shared x axis.** The rows are the rows of **one `plot::Figure`**, drawn by one
 `plot::gtk4::PlotArea` in figure mode, and they all join a single x link group.

src/channel.hpp

@@ -73,8 +73,8 @@ class ChannelBuffer final : public plot::data::Series {
   /// datagram leaves a gap, so index times must come from the time ring.
   f64 x_at(u64 i) const override;
   /// Bracketing samples by binary search over the time ring, which is monotonic,
-  /// so a crosshair value is interpolated between the two samples actually either
-  /// side of the pointer -- a grid walk would land on the wrong pair after a gap.
+  /// so a crosshair reads whichever of the two samples actually either side of the
+  /// pointer is nearer -- a grid walk would land on the wrong pair after a gap.
   bool neighbours(f64 x, u64& i0, u64& i1) const override;
   u64 snap_index(f64 x) const override;
   plot::Range x_bounds() const override;

src/help/05-window.txt

@@ -19,6 +19,9 @@ unit, width-locked so a change of window cannot widen the header bar.
   value. Stamping at the end of a group would shift every trace half a stored sample late.
 • Moving the window across a decimation boundary rebuilds the buffer and drops the history.
   Walking it inside one decade is free.
+• <b>This control is the only thing that changes the window.</b> A zoom, a pan or a fit moves the
+  view <i>inside</i> it and leaves it alone — which is the point of the previous bullet: a wheel
+  notch that redefined the window could re-plan the buffer and throw your samples away.
 • The trigger sees the <i>stored</i> (filtered) signal, not the wire signal, because an edge
   found on a sample that is not in the buffer could not be cut out of it.
 

src/help/06-navigate.txt

@@ -19,6 +19,11 @@ The full table is in the shortcut overview (<tt>Ctrl+/</tt>).
 • A <i>pinned</i> view is navigable within its own data but cannot leave it. While an event is
   held, a capture is filling, or the view is paused, a zoom <i>in</i> works normally — "not
   outside the event" is not "not readable" — but a pan or a zoom out is clamped to the data.
+• Nothing you do on the plot changes the <i>time window</i> — only the header control does. A
+  zoom or a pan moves the view inside the window; <b>f</b> (and Fit X axis) is the <i>undo</i> of a
+  zoom, so a 2 s window zoomed into reads 2 s again. The window is more than the axis: it is also
+  the trigger's event span and how the samples are stored, so a wheel notch that redefined it
+  would re-plan the buffer and drop your history.
 • A live view is clamped to the window you asked for, not to what the ring happens to hold.
   That is why a buffer that has only just started filling leaves its left-hand side empty
   instead of squeezing the window down to the first few samples.

src/help/09-cursors.txt

@@ -19,10 +19,22 @@ owns a <b>Reset</b> button. Drag a marker to move it. The mode dropdown offers f
 • <tt>mix</tt> is the reading mode: one line per <i>bound</i> trace, with the trace's own values
   at each marker and their difference. A slot with a name but no channel behind it is not a
   trace and gets no line.
+• A value is the <i>nearest stored sample's</i>, never a number between two samples. Zoom in past
+  the sample spacing and the line between two samples is something the plot drew, not data —
+  so a reading off it would be the drawing. The hover crosshair reads the same way.
+• Every reading carries a <i>metric prefix</i> so the digits sit in <tt>1..999</tt>:
+  <tt>10.52 mV</tt>, not <tt>0.01 V</tt>. Two decimals is all an instrument landing on an
+  <tt>f32</tt> sample can claim, so without the prefix a small value would read <tt>0.00</tt>
+  and the reading would be gone. A stream that declares no unit gets the prefix anyway, written
+  tight against the number (<tt>585.24m</tt>) — the space separates the number from its unit,
+  and a spaced <tt>m</tt> with nothing after it would read as metres.
 • A marker past the newest sample reads <tt>--</tt>. With only one marker over data the row
   reads <tt>partial</tt>: a difference between a number and nothing is not a number.
 • In <tt>mix</tt> each marker also places a crossing annotation per trace, which is what lets
-  you see on the plot which sample a number came from.
+  you see on the plot which sample a number came from. It is <i>on that sample</i>, so when the
+  marker falls between two it sits a little off its own line: the line is where you put the
+  marker, the dot is the datum being quoted. The hover crosshair's mark snaps the same way, and
+  is drawn larger than a trace's sample dots so it does not read as one more sample.
 • The time row exists only in the modes that show the vertical pair. <tt>horizontal</tt> has no
   time row, by design.
 

src/help/10-measurements.txt

@@ -27,7 +27,11 @@ set, with <b>Save</b> and <b>Delete</b>.
 • A window with too few crossings reads <tt>invalid</tt> / <tt>--</tt> rather than a plausible
   invented number. Frequency over a window holding one edge is the usual case.
 • <tt>frequency</tt> is reported in Hz and <tt>period</tt>/<tt>pulse duration</tt> in seconds;
-  every other type carries the channel's own unit.
+  every other type carries the channel's own unit. A value is written with a <i>metric prefix</i>
+  so its digits sit in <tt>1..999</tt> — <tt>636.62 mV</tt>, not <tt>0.64 V</tt> — including when
+  the stream declares no unit at all, which is always the case for a math channel: there the
+  prefix goes tight against the number (<tt>636.62m</tt>), since two decimals of an unprefixed
+  small value is <tt>0.00</tt>.
 • Cost is bounded by reading the summary pyramid the producer already maintains, not the raw
   window. The values are still exact: a bucket's sum is a sum.
 

src/plot_host.cpp

@@ -396,18 +396,30 @@ void RowSet::place_crossings() {
       if (ida >= r.plot->annotation_count() || idb >= r.plot->annotation_count()) continue;
       plot::Annotation a = r.plot->annotation(ida);
       plot::Annotation b = r.plot->annotation(idb);
+      double xa = 0.0;
       double ya = 0.0;
+      double xb = 0.0;
       double yb = 0.0;
-      const bool has_a = mix && r.plot->crosshair_value(static_cast<plot::u32>(s), cursor_a_x_, ya);
-      const bool has_b = mix && r.plot->crosshair_value(static_cast<plot::u32>(s), cursor_b_x_, yb);
+      // `crosshair_point()`, not `crosshair_value()`: the number beside this dot is a *stored
+      // sample's*, so the dot belongs on that sample and not on the marker's own column. The
+      // two differ by up to half a sample period, which is nothing at a whole-window view and
+      // the whole width of the panel once the view is zoomed to a handful of samples -- the
+      // state in which a reader is asking "which point is that?" in the first place. It leaves
+      // the dot a little off its vertical line by design: the line says *where the marker is*
+      // and the dot says *which datum is being quoted*, and when the marker is between two
+      // samples those are not the same place.
+      const bool has_a =
+          mix && r.plot->crosshair_point(static_cast<plot::u32>(s), cursor_a_x_, xa, ya);
+      const bool has_b =
+          mix && r.plot->crosshair_point(static_cast<plot::u32>(s), cursor_b_x_, xb, yb);
       a.visible = has_a;
       b.visible = has_b;
       if (has_a) {
-        a.x = cursor_a_x_;
+        a.x = xa;
         a.y = ya;
       }
       if (has_b) {
-        b.x = cursor_b_x_;
+        b.x = xb;
         b.y = yb;
       }
       r.plot->set_annotation(ida, a);
@@ -646,20 +658,26 @@ void RowSet::unpin_pause_window() {
 }
 
 void RowSet::fit_x() {
-  Row& first = row(0);
-  if (!first.plot || !figure_) return;
-  first.plot->auto_fit_x();
-  const plot::Range fitted = first.plot->x_range(first.axis_x);
-  const double span = fitted.span();
-  if (!std::isfinite(span) || !(span > 0.0)) return;
-  // The same two paths `fit_all()` takes: a held event owns its window and states it
-  // through the link group, a live view re-arms the follow and frames the newest span.
-  if (pinned_) {
-    window_s_ = span;
-    figure_->set_x_link_range(x_group_, fitted);
-  } else {
-    set_time_window(span);
+  if (!figure_) return;
+  if (!pinned_) {
+    // Live: the same path the window control and a resume take, with the window it already has.
+    // That re-arms the follow and parks the newest `window_s_` at the live edge, which is the
+    // whole of what a live view may show.
+    set_time_window(window_s_);
+    return;
   }
+  // Pinned (a held event or a paused view): the pin's full range, and the pin stays. The
+  // *members* have to be written as well as the group, because `hold_pinned_window()` prefers a
+  // member that has moved since our own last statement -- that is how it tells a gesture from its
+  // own echo -- so writing the group alone would be reverted to the zoomed-in member next frame.
+  const plot::Range pin{-pinned_pre_, pinned_post_};
+  if (!(pin.span() > 0.0) || !std::isfinite(pin.min) || !std::isfinite(pin.max)) return;
+  for (int i = 0; i < rows_; ++i) {
+    Row& r = row(i);
+    if (r.plot) r.plot->set_x_range(r.axis_x, pin);
+  }
+  pinned_stated_ = pin;
+  figure_->set_x_link_range(x_group_, pin);
 }
 
 void RowSet::sync_cursors(int drag_row) {
@@ -778,23 +796,9 @@ void RowSet::fit_all() {
     Row& r = row(i);
     if (r.plot) r.plot->auto_fit_y(r.axis_y);
   }
-  Row& first = row(0);
-  if (!first.plot || !figure_) return;
-  // Row 0 fits its own x to the data it holds; that span becomes the shared window, and
-  // stating it through the group is what puts every row on it.
-  first.plot->auto_fit_x();
-  const plot::Range fitted = first.plot->x_range(first.axis_x);
-  const double span = fitted.span();
-  if (!std::isfinite(span) || !(span > 0.0)) return;
-  if (pinned_) {
-    // Fitting a captured event: the window is the snapshot's own `[-pre, +post]` span,
-    // and `set_time_window()` would re-arm the follow and re-frame on a live edge that
-    // trigger mode has nothing bound to.
-    window_s_ = span;
-    figure_->set_x_link_range(x_group_, fitted);
-  } else {
-    set_time_window(span);
-  }
+  // The x half is `fit_x()`'s exactly -- the zoom undone, the time window untouched -- and
+  // sharing it is what keeps the two fits from drifting apart.
+  fit_x();
 }
 
 void RowSet::note_y_gutter(double need_px) {

src/plot_host.hpp

@@ -298,11 +298,25 @@ class RowSet {
   void set_data_pin(double lo, double hi);
   void unpin_pause_window();
 
-  /// Frames the x window on the data row 0 holds, leaving every y axis alone. The x
-  /// counterpart of `fit_all()`, for the one thing the y axes cannot say: how wide the
-  /// window should be.
+  /// Undoes an x zoom: puts the view back on the whole time window, leaving every y axis alone.
+  /// The x counterpart of `fit_all()`.
+  ///
+  /// **It does not change the time window** -- `window_s_` is read, never written. A fit is a
+  /// plot interaction, and the window is also the trigger's event span, the buffer partition and
+  /// the control's readout, so a fit that *framed the data* instead would redefine all of that
+  /// from a gesture (and re-plan the rings, dropping the history). What a user asks for here is
+  /// the zoom undone: a 2 s window zoomed into reads 2 s again.
+  ///
+  /// Which 2 s depends on the state, and both cases restore the *whole* of what the view is
+  /// allowed to show: live, the newest window at the live edge; pinned (a held event or a paused
+  /// view), the pin's full range, with the pin kept -- `set_time_window()` would clear it and
+  /// re-frame on a live edge that a frozen view has nothing bound to.
   void fit_x();
 
+  /// Whether a window is pinned: a held event, a filling capture or a paused view. The condition
+  /// `fit_x()` branches on, which is why the fit probe reports it.
+  bool window_pinned() const noexcept { return pinned_; }
+
   /// Adopts the instruments a gesture is moving, on `drag_row` (the row whose controller holds
   /// a cursor), and nothing otherwise.
   ///
@@ -336,10 +350,9 @@ class RowSet {
   /// 0, so nothing outside the pre+post window is visible.
   void pin_event_window(double pre_s, double post_s);
 
-  /// Fits every row's y axis to the data it holds, and re-frames the shared x window
-  /// on what row 0 retains (the same thing the plot's own `f` key does). The window
-  /// the toolbar reports becomes the fitted span, so the dropdown can say what is
-  /// actually on screen.
+  /// Fits every row's y axis to the data it holds, and puts x back on the whole time window --
+  /// `fit_x()`'s x half exactly, so the two cannot drift apart. Like it, **this does not change
+  /// the time window**: the y axes are fitted to their data, x is merely un-zoomed.
   void fit_all();
 
   /// Reports the left gutter the rows currently need, and reserves at least the width

src/ui.cpp

@@ -922,6 +922,13 @@ const char* badge_markup(BadgePhase phase) {
 /// sample is an `f32` of a value the CONFIG may scale arbitrarily, so more digits would be
 /// noise dressed as precision. Trailing zeros go, so `1.50 V` reads `1.5 V` -- "at most
 /// two decimals", not exactly two.
+///
+/// The prefix is applied **whether or not a unit is declared**, and that is the point: two
+/// decimals of an unprefixed small value is `0.00`, i.e. the reading is simply gone. A signal
+/// whose CONFIG declares no unit is ordinary (and a *math* channel never has one), so the pane
+/// showed `0.00` for every value under 5 mV of an entire class of channels. A bare prefix is
+/// written tight against the number -- `1.34m`, not `1.34 m` -- because the space is there to
+/// separate the number from its unit, and with no unit to separate, a spaced `m` reads as metres.
 std::string format_measure(double v, const std::string& unit) {
   if (!std::isfinite(v)) return "--";
   static const struct {
@@ -934,22 +941,22 @@ std::string format_measure(double v, const std::string& unit) {
   const double mag = std::fabs(v);
   const char* prefix = "";
   double scale = 1.0;
-  // No unit, no prefix: `10.52 m` is a prefix with nothing to prefix, which reads as
-  // metres. A bare number stays a bare number and simply keeps its decimals.
-  if (!unit.empty()) {
-    for (const auto& p : kPrefixes) {
-      if (mag >= p.scale) {
-        prefix = p.prefix;
-        scale = p.scale;
-        break;
-      }
-    }
-    if (mag > 0.0 && mag < 1.0e-12) {  // below the ladder: keep it in plain digits
-      prefix = "";
-      scale = 1.0;
+  for (const auto& p : kPrefixes) {
+    if (mag >= p.scale) {
+      prefix = p.prefix;
+      scale = p.scale;
+      break;
     }
   }
   char buf[64];
+  if (mag > 0.0 && mag < 1.0e-12) {
+    // Below the ladder, where scaling cannot help and `%.2f` would print `0.00` again. Three
+    // significant digits in whatever form fits, so the reading survives as a reading.
+    std::snprintf(buf, sizeof(buf), "%.3g", v);
+    std::string out = buf;
+    if (!unit.empty()) out += " " + unit;
+    return out;
+  }
   std::snprintf(buf, sizeof(buf), "%.2f", v / scale);
   std::string num = buf;
   if (num.find('.') != std::string::npos) {  // at most two decimals, not exactly two
@@ -958,11 +965,9 @@ std::string format_measure(double v, const std::string& unit) {
   }
   if (num == "-0") num = "0";
   std::string out = num;
-  if (prefix[0] != '\0') out += std::string(" ") + prefix;
-  if (!unit.empty()) {
-    if (prefix[0] == '\0') out += " ";
-    out += unit;
-  }
+  if (!unit.empty()) out += " ";  // the space separates the number from its unit, nothing else
+  out += prefix;
+  out += unit;
   return out;
 }
 
@@ -1865,10 +1870,19 @@ void sync_window_display(App& app, double seconds) {
   app.updating_panels = false;
 }
 
-/// The one place the display window changes. Clamps to what the buffer can hold, moves the
-/// plot and the trigger's event span together -- its pre-trigger share is a share of *this*
-/// window -- and then restates the result in the control, so a clamped request reads back as
-/// the window that was actually taken rather than the one that was typed.
+/// The one place the time scale changes. Clamps to what the buffer can hold, moves the plot and
+/// the trigger's event span together -- its pre-trigger share is a share of *this* window -- and
+/// then restates the result in the control, so a clamped request reads back as the window that
+/// was actually taken rather than the one that was typed.
+///
+/// **Only the window control comes here.** The time scale is more than the axis: it is also the
+/// trigger's event span, the buffer partition (so a change of decimation drops the history) and
+/// the session, so a *plot interaction* may not state one. A zoom and a pan never touch it by
+/// construction -- they move the row's axis, and `clamp_to_data()` caps the span at `window_s_` --
+/// and the two fits do not either: `RowSet::fit_x()` undoes a zoom by restoring the window that
+/// is already set. Both fits used to frame the data instead and move `window_s_` alone, which
+/// left the event span on the *old* window until the next decade step or unit change applied the
+/// fitted value for real: one action, two changes of time scale.
 void apply_time_window(App& app, double seconds) {
   if (!(seconds > 0.0)) return;
   const double clamped = std::min(std::max(seconds, kWindowMinS), window_limit(app));
@@ -1882,8 +1896,9 @@ void apply_time_window(App& app, double seconds) {
                  format_rate(app.model->partition().stored_rate()) + ", history cleared";
     app.notice_until_us = g_get_monotonic_time() + 4 * G_TIME_SPAN_SECOND;
   }
-  app.rows->set_time_window(clamped);
   app.model->set_event_span(clamped);
+  // The partition before the axis, so the axis is framed over the rings that will exist.
+  app.rows->set_time_window(clamped);
   sync_window_display(app, clamped);
   schedule_layout_save(app);
 }
@@ -4046,25 +4061,25 @@ void on_signals_toggled(GtkToggleButton* button, gpointer user) {
                                 gtk_toggle_button_get_active(button));
 }
 
-/// Fit: auto-scale every row's y axis and re-frame the x window on the data the
-/// ring holds (the plot's own `f` semantics), then tell the toolbar what span that
-/// is.
+/// Fit: auto-scale every row's y axis to the data it holds, and put x back on the whole time
+/// window.
+///
+/// No `apply_time_window()` here, deliberately: a fit is a plot interaction and may not restate
+/// the time scale (see that function). The y axes are fitted to the data; x is only un-zoomed.
 void on_fit_clicked(GtkButton*, gpointer user) {
   App* app = static_cast<App*>(user);
   app->rows->fit_all();
-  sync_window_display(*app, app->rows->time_window());
-  app->notice = "fitted to the buffered data (" + format_window(app->rows->time_window()) + ")";
+  app->notice = "y fitted to the data, x back to the " +
+                format_window(app->rows->time_window()) + " window";
   app->notice_until_us = g_get_monotonic_time() + 3 * G_TIME_SPAN_SECOND;
 }
 
-/// Fit the x window only: the y axes keep the scales the user set. The counterpart of the
-/// other fit, for the common case where the interesting range is the data's but the levels
-/// were placed by hand.
+/// Reset the x zoom only: the y axes keep the scales the user set. The counterpart of the other
+/// fit, for the common case where a zoom should be undone but the levels were placed by hand.
 void on_fit_x_clicked(GtkButton*, gpointer user) {
   App* app = static_cast<App*>(user);
   app->rows->fit_x();
-  sync_window_display(*app, app->rows->time_window());
-  app->notice = "x fitted to the buffered data (" + format_window(app->rows->time_window()) + ")";
+  app->notice = "x back to the " + format_window(app->rows->time_window()) + " window";
   app->notice_until_us = g_get_monotonic_time() + 3 * G_TIME_SPAN_SECOND;
 }
 
@@ -7176,15 +7191,21 @@ void cursors_probe_report(App& a, const char* what) {
 gboolean cursors_probe_step(gpointer user);
 
 /// The two fits the right-click menu offers, driven through the handlers the menu wires
-/// (a null button: neither handler looks at it). `fit x` must narrow the window to the
-/// data's own span and leave the y ranges alone; `fit all` must then move the y ranges too.
+/// (a null button: neither handler looks at it). Both must put x back on the whole time window
+/// and **leave the window itself alone**; `fit x` must leave the y ranges alone too, and
+/// `fit all` must move them to the data's.
 void fit_probe_report(App& a, const char* what) {
   const RowSet::Row& r0 = a.rows->row(0);
   const plot::Range x = r0.plot != nullptr ? r0.plot->x_range(r0.axis_x) : plot::Range{};
   const plot::Range y = r0.plot != nullptr ? r0.plot->y_range(r0.axis_y) : plot::Range{};
+  // `window` and `event_span` are the assertion that a fit did *not* restate the time scale:
+  // both are written only by `apply_time_window()`, so they must hold the typed value on every
+  // line, with `span` the reading that moves -- down on the zoom, back up on the fit.
   std::fprintf(stderr,
-               "[fit-probe] %-10s window=%.6f x=[%.6f..%.6f] span=%.6f y=[%.6f..%.6f]\n",
-               what, a.rows->time_window(), x.min, x.max, x.span(), y.min, y.max);
+               "[fit-probe] %-10s pinned=%d window=%.6f event_span=%.6f x=[%.6f..%.6f] span=%.6f "
+               "y=[%.6f..%.6f]\n",
+               what, a.rows->window_pinned() ? 1 : 0, a.rows->time_window(), a.model->event_span(),
+               x.min, x.max, x.span(), y.min, y.max);
   std::fflush(stderr);
 }
 
@@ -7261,32 +7282,57 @@ gboolean timelabels_probe_step(gpointer user) {
   return G_SOURCE_REMOVE;
 }
 
+/// Zooms row 0 in to a quarter of what it shows, the way a gesture does -- on the *member* axis,
+/// which `Figure::layout()` adopts into the group. A fit is the undo of this, so the probe has to
+/// perform it or it has nothing to undo.
+void fit_probe_zoom_in(App& a) {
+  RowSet::Row& r0 = a.rows->row(0);
+  if (r0.plot == nullptr) return;
+  const plot::Range x = r0.plot->x_range(r0.axis_x);
+  const double mid = x.min + 0.5 * x.span();
+  const double half = 0.125 * x.span();
+  r0.plot->set_x_range(r0.axis_x, {mid - half, mid + half});
+}
+
 gboolean fit_probe_step(gpointer user) {
   App* a = static_cast<App*>(user);
   if (a->window == nullptr) return G_SOURCE_REMOVE;
   switch (a->dbg_fit_step) {
     case 0:
-      // Move the y scale off the data's range first, so "fit x leaves y alone" is a
-      // statement with something to be wrong about.
+      // Move the y scale off the data's range first, so "fit x leaves y alone" is a statement
+      // with something to be wrong about -- and set the window through the control's own path,
+      // since that is the only thing allowed to, and its value is what both fits must preserve.
       a->rows->set_y_range(0, -100.0, 100.0);
-      a->rows->set_time_window(0.05);
+      // Not while pinned: `apply_time_window()` unpins, and the pinned branch of the fit is the
+      // other half of what this probe is for. Timed inside a pause (`UDPSCOPE_PROBE_PAUSE_MS`)
+      // it takes the window the pause froze and reports `pinned=1`.
+      if (!a->rows->window_pinned()) apply_time_window(*a, 2.0);
       fit_probe_report(*a, "start");
       g_timeout_add(400, fit_probe_step, a);
       a->dbg_fit_step = 1;
       break;
     case 1:
-      a->rows->fit_x();
-      sync_window_display(*a, a->rows->time_window());
+      fit_probe_zoom_in(*a);
       g_timeout_add(400, fit_probe_step, a);
       a->dbg_fit_step = 2;
       break;
     case 2:
-      fit_probe_report(*a, "fit x");
-      a->rows->fit_all();
-      sync_window_display(*a, a->rows->time_window());
+      // A quarter of the window: what the fits have to undo, and the reading that proves the
+      // `span=` on the fit lines is the fit's work rather than a window nothing ever left.
+      fit_probe_report(*a, "zoomed");
+      // The handlers themselves, not `RowSet::fit_x()` alone: the window the fit must *not*
+      // move is the app's, and the notice and the control are the handler's half.
+      on_fit_x_clicked(nullptr, a);
       g_timeout_add(400, fit_probe_step, a);
       a->dbg_fit_step = 3;
       break;
+    case 3:
+      fit_probe_report(*a, "fit x");
+      fit_probe_zoom_in(*a);
+      on_fit_clicked(nullptr, a);
+      g_timeout_add(400, fit_probe_step, a);
+      a->dbg_fit_step = 4;
+      break;
     default:
       fit_probe_report(*a, "fit all");
       break;

tools/mock_udpstreamer.py

@@ -50,16 +50,16 @@ def signal_desc(name, *, rate, time_mode, time_idx, unit, rows=1, cols=1, dims=0
     return bytes(d)
 
 
-def build_config(rate, time_mode=TIME_FULL_ARRAY):
+def build_config(rate, time_mode=TIME_FULL_ARRAY, unit="V"):
     sigs = [
         # The stream's clock: referenced by every data signal, so udpscope treats
         # it as the time base rather than a plottable channel. Its *own* time_mode
         # is what udpscope switches on, so `--time-mode first` has to land here.
         signal_desc("time", rate=0.0, time_mode=time_mode,
                     time_idx=NO_TIME_SIGNAL, unit="s"),
-        signal_desc("sine", rate=rate, time_mode=time_mode, time_idx=0, unit="V"),
-        signal_desc("square", rate=rate, time_mode=time_mode, time_idx=0, unit="V"),
-        signal_desc("ramp", rate=rate, time_mode=time_mode, time_idx=0, unit="V"),
+        signal_desc("sine", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
+        signal_desc("square", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
+        signal_desc("ramp", rate=rate, time_mode=time_mode, time_idx=0, unit=unit),
     ]
     payload = struct.pack("<I", len(sigs)) + b"".join(sigs)
     payload += bytes([PUBLISH_ACCUMULATE])
@@ -101,6 +101,10 @@ def main():
                          "sample (UDPS_TIME_FIRST_SAMPLE) and leaves the viewer to space the "
                          "rest by the declared cadence -- the shape a real MARTe2-style "
                          "producer with a single u64 clock has")
+    ap.add_argument("--unit", default="V",
+                    help="the unit the data signals declare in CONFIG; '' declares none, which "
+                         "is an ordinary stream (and what every math channel looks like) and is "
+                         "the only way a probe can see the unitless readout formatting")
     ap.add_argument("--clock-jitter", type=float, default=0.0,
                     help="per-packet stamp jitter, as a fraction of one packet's span: a device "
                          "that stamps each batch from its own clock does not land on a perfect "
@@ -138,7 +142,7 @@ def main():
                     t_stream = 0.0
                     started = time.monotonic()
                     next_send = started
-                    sock.sendto(build_config(declared, time_mode), peer)
+                    sock.sendto(build_config(declared, time_mode, args.unit), peer)
                     print("mock udpstreamer: CONNECT from %s:%d, CONFIG sent" % peer, flush=True)
                 elif magic == MAGIC and ptype == PKT_DISCONNECT:
                     print("mock udpstreamer: DISCONNECT", flush=True)