added auto measuraments

CLAUDE.md

@@ -201,10 +201,94 @@ Two lines of the self-check output are assertions in disguise:
   2 kHz at about 6%. **A `y=[-1, 1]` there means the filter is gone** and the 2 kHz sine has
   aliased to a full-amplitude 125 Hz one, which looks entirely plausible on screen. This is
   the only check that distinguishes averaging from striding.
-- `[layout-probe] <action> rows=N buttons=B rows_y=[..] close=[..] add=[..] traces=[..]`
+- `[cursors-probe] <action> mode=.. on=D pane=D t0=.. t1=.. dt=.. T=[..] lines=N [..]`
+  (stderr, `UDPSCOPE_PROBE_CURSORS_MS`) — walks `vertical` → `horizontal` → `both` →
+  `mix`, then moves a marker *through the plot* (`Plot::set_cursor_x(..., Free)`, which is
+  what a drag does) and reports again. **The times must survive every mode change**: they
+  are the `RowSet`'s one pair, and a mode that parks the markers must not read them back
+  (a parked cursor reports a real number, so a read-back would adopt it as a gesture — the
+  bug that made `T0` read 0). **`lines`** must be 0 for `vertical`, one per row for
+  `horizontal`/`both` (and the time row exists only in the modes that show the vertical
+  pair — `T=[]` in `horizontal` is the assertion that it does not), and one per bound trace
+  for `mix`; in `mix` the values must be the
+  traces' own (`--` when a marker is past the newest sample, `partial` when only one
+  marker is over samples). After the move, `t0` must have changed and every row's marker
+  must have followed it (the report's `t0` is the shared value). **`marks`** is the `mix`
+  mode's crossing annotations: **0 in every other mode, and one per bound trace for each
+  marker that is over samples in `mix`** (so a move that brings the second marker into the
+  data doubles it). The `first mark at x=.. y=..` line that follows must agree with the
+  pane's *first* value for the same trace (the pane's lines are `chip  Y1  Y2  dY`, with
+  the values formatted as measurements -- two decimals, an SI prefix, the channel's unit --
+  so the comparison is against the first number): both are read through the same
+  `Plot::crosshair_value()`, and that line is the check that they stay the same number.
+  A **ruler** drag is the other direction and the other hazard: the pane's `A`/`B` cells
+  for a ruler row must change when the ruler moves (`after ruler drag` in the probe walks
+  the first ruler to 80% of its y range), because nothing shares a ruler and the read-back
+  in `RowSet::sync_cursors()` is the only thing that can notice. The dump's `cursors in plot
+  0:` line is the other half: it reads the markers back out of
+  the plot (`cursor(id).x/.visible`), which is what the renderer draws from — the pane's
+  numbers and the plot's state agreeing is the difference between "the app placed them" and
+  "the app placed them and they are on screen".
+- `[measures-probe]` + the dump's `measure N: 'sig' type thr=..% above=D show=D -> ok|invalid value text=[..]
+  [duty=..] [marks=..]` and `measures pane: n=N revealed=D marks_shown=M pane_rows=R pane_w=W
+  pane_h=H`
+  (stderr, `UDPSCOPE_PROBE_MEASURES_MS`) — adds one measurement of every type on the first
+  plottable signal and lets the tick compute them, so what it prints is what a user reads.
+  On the test stream's 1 Hz ±1 sine the values must be: average ≈ 0, `top` ≈ 0.64 and
+  `bottom` ≈ -0.64 (the mean of each half of a sine), max/min ≈ ±1, `pulse duration` ≈ 0.5 s
+  with `duty` ≈ 0.5, `period` ≈ 1 s and `frequency` ≈ 1 Hz. `marks=` lists each mark with its kind, and the **mix is the point**:
+  `level:V` alone for average/top/bottom, `level:V,time:T` for min/max (a ruler at the value
+  *and* a marker at the first sample that reached it — two level marks at both extremes was
+  the bug), `time:A,time:B` for pulse/period/frequency. `text=` is the pane's own string with
+  its unit, so `frequency` must read `Hz` (not the signal's unit), `period`/`pulse duration`
+  seconds, and the rest the channel's unit. `marks_shown` counts two per shown measurement
+  (a min/max showing both its marks counts two of its own). `measures window:` is the
+  timeliness assertion: **`drift` must stay at (or near) zero** -- the window the numbers
+  were computed over against the one on screen -- and every *time* mark must lie inside it
+  (`marks_in_window=K/K`). A measurement is recomputed when the window moves (2 % of its
+  span) or the held/live state changes, never on a wall-clock timer: a timer is what let a
+  marker sit outside the viewport and a dragged one survive a frame. **`marks_selectable` must be 0**: the
+  measurement marks are created unselectable (`Plot::set_cursor_selectable()`, which the
+  library gained for this), read back out of the plot here -- re-placing a marker every frame
+  does **not** stop a drag, because the controller holds it for the gesture's life and the
+  marker follows the pointer between frames. The library's own controller test
+  (`an_unselectable_cursor_is_neither_hovered_nor_grabbed`) is the other half. The probe's last
+  step still drags a mark *programmatically* and requires the next frame to put it back
+  (`restored=1`), which now checks the placement path rather than the read-only property. The dialog report's `type= thr=
+  side= show=` is the per-type behaviour: `period` and `frequency` must show **neither** the
+  split nor the side row (they compute from the mid level's rising crossings, and an option
+  there is one the user can only get wrong), `average`/`min`/`max` no split either, and only
+  `pulse duration` a side. A measurement whose signal is not acquired, or
+  a window with too few crossings, must read `invalid`/`--` rather than a made-up number
+  (`frequency` on a window holding one edge is the case: `duty` is reported only when a
+  period exists to divide by). The probe then **walks the dialog's lifecycle**: open the edit
+  dialog on measurement 0, click its `Save`, open it again, click `Delete` — `measures=` must
+  be unchanged after the save and one lower after the delete, with the process still alive.
+  That is the check for the two things a dialog of this shape gets wrong: two buttons owning
+  one context (a double free aborts on edit/delete, and only those paths have two buttons) and
+  a `const char*` array built from a `std::vector<std::string>` as it grew (every pointer
+  dangling, the first entry the most visibly). The report's `signals=[..]` and `drop_h` are the second and third:
+  the list must be the **plotted traces** (real names, deduplicated, in signal-list order —
+  *not* every channel, and never a stream's timestamp signal, which is what an acquired-but-
+  unplotted list put first and auto-selected), and `drop_h` must be a combo box's height
+  (tens of pixels — a scrolled-window wrapper asked it to grow to 160).
+- `[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.
+- `[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
-  buttons exist only while the editor does. **`traces=` is the real assertion**: an insert
+  buttons exist only while the editor does. `done` follows the same rule for the editor's
+  own exit button (1 up, 0 off), and its `done_box` on the `placed` line must sit in the
+  area's top-right corner: `x + W` within a few pixels of the area width and `y` within a
+  few of 0 (the on-switch line reports a box from before the first frame, so read the
+  `placed` one). The last probe step leaves the editor by **clicking that button** — the
+  toolbar toggle is still exercised by the first step — so `buttons=0 done=0` with the
+  rows and traces untouched is the assertion that the way out works. **`traces=` is the real assertion**: an insert
   or a remove moves the rows' *contents* with them, so `add above 0` must read
   `[0:-|1:sine,…]` — row 0's signals now on row 1, not renumbered onto the new empty row —
   and `remove 0` must pull them back to `0:`. A `traces=` where the signals stayed on row 0
@@ -229,6 +313,14 @@ Two lines of the self-check output are assertions in disguise:
   first run's values, and the signals it names must come back *acquired* (a `row 0 slot 0
   -> channel ... samples=N` line). Delete the file before a run whose layout you did not
   mean to inherit (`127.0.0.1_9540.toml` and friends accumulate otherwise).
+- `[udpscope] starved: connected (config=D), reconnects=N, no new frames in the last
+  second ...` (stderr, once per second while the state holds, plus a status-line notice) —
+  "connected but nothing is arriving after a reconnect", which over unicast is the second
+  instance taking the source's single session. Two ways to see it without a second client:
+  point the app at a UDP port where nothing answers (the client CONNECTs, times out,
+  reconnects; `config=0`), and check it does **not** fire against a live source. The status
+  line carries ` | N reconnects` only when N > 0, which is the same counter without the
+  prose.
 - **The export's `time`/`stream_time` columns must be strictly increasing and evenly
   spaced.** Check the written file, not just the probe: `np.diff(t)` with no `<= 0` and
   `rows / (t[-1] - t[0])` equal to the stored rate (the `ch` line's figure). Duplicate or
@@ -363,6 +455,13 @@ 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.
+- **The cursors' on/off is one flag on the `RowSet`, never a copy per row.** It is one
+  piece of state (the toolbar toggle) and the mode is one value; a per-row copy is something
+  `insert_row()`'s `clear_row()`, or any rebuild, can clear behind the user's back, which is
+  how a row added while the cursors were already on came up with its instruments parked
+  while every other row showed them. The same goes for the vertical pair's `x`: it is one
+  stream time for the figure, so it lives on the `RowSet` too, and `Row` keeps only what is
+  genuinely per row (the rulers' values, the traces, the colours).
 - **A row's contents are state of the `RowSet`, not of the figure.** `insert_row()` and
   `remove_row()` shift that state with `carry_row()` and let `rebuild_all()` re-deal the
   figure handles, so inserting above row 1 moves what row 1 plotted down to row 2 rather

CMakeLists.txt

@@ -41,6 +41,7 @@ add_executable(udpscope
   src/plot_host.cpp
   src/export.cpp
   src/layout_store.cpp
+  src/measure.cpp
   src/ui.cpp
 )
 

README.md

@@ -58,6 +58,68 @@ frame and the GUI thread takes once per frame around binding the traces.
 Samples cross the boundary through `plot::data::RingBuffer` (single producer,
 single consumer), never through the lock.
 
+## Measurements
+
+**Measurements** (overflow menu) reveals a pane with a `[+]` and one row per measurement:
+`[badge] <type> <value> [edit]`. `[+]` opens a dialog with the signal, the type, the options
+that type actually uses, and "show on plot"; `edit` opens the same dialog on that
+measurement, with a `Delete` beside `Save`.
+
+The signal list is the signals the **plots are showing**, in signal-list order: a
+measurement reads the window a plot displays and its marks go on the row that plots the
+signal, so the candidates are the traces on screen. That is also what keeps everything else
+out of it — a signal nobody acquires, and a stream's own timestamp channel, which is acquired
+but is not something to measure. A measurement whose signal is no longer plotted keeps it in
+the list while it is being edited, so saving does not silently re-point it at whatever the
+dropdown happened to be showing. The badge is the measurement's own colour, and
+the marks it draws on the plot use it, so a readout and its ink are recognisably the same
+measurement.
+
+| type | what it is | options | shown on the plot as |
+|---|---|---|---|
+| `average` | the mean of the visible samples | — | a ruler at the mean |
+| `top` / `bottom` | the high / low level: the mean of the samples above / below the split | split % | a ruler at the level |
+| `max` / `min` | the extremes | — | a ruler at the value **and** a marker where it happened |
+| `pulse duration` | the mean width of the pulse-side runs | split %, which side is the pulse | markers at the pulse's two edges |
+| `period` / `frequency` | the spacing of the threshold crossings / its reciprocal, in Hz | — | markers at two edges of a cycle |
+
+`period` and `frequency` deliberately take **no options**: the engine counts *rising*
+crossings of the *mid* level, one edge per cycle, and a split or a side to choose from could
+only make that count wrong — a split near either extreme counts noise as crossings, and
+counting both edges halves the period.
+
+The split is a share of the **window's own** min..max, not volts: a measurement is about this
+signal in this window, and a level in volts would have to be typed for every signal. A
+crossing has to be *strict* — a sample exactly at the level is on neither side — so a signal
+that merely touches the level does not count every sample as an event.
+
+Each measurement's row carries **two badges**: its own colour (the same one its marks use on
+the plot) and the signal's chip, in the palette colour every other view names that signal
+with. The dialog's fields and their wording follow the type — the split row is absent for
+`average`, `min` and `max`, the side row only for `pulse duration`, and the "show on plot"
+line says what would be drawn.
+
+The values are **window statistics over exactly the visible range, at the stored rate**: the
+same cut the export makes, so a measurement describes the data on screen rather than the ring
+behind it — and the marks it draws belong to that window, so they stay in the viewport.
+
+They are recomputed when the **window moves**, not on a timer: a timer decouples the two, and
+with a scrolling window the stored marks then describe a range the plot has already left (the
+min/max marker ends up outside the viewport, and a dragged marker survives until the next
+tick). The trigger is 2 % of the span, so the numbers track the window within a few pixels
+while the cost stays bounded by the window's motion rather than by the frame rate; one pass
+copies the visible samples of each *channel* once, however many measurements read it.
+
+**Show on plot** marks the result on the row that plots the signal: a ruler at the level
+(`top`/`bottom`), both rulers at the extremes (`min`/`max`), the pulse's or the cycle's two
+edge times as vertical markers. The marks are **readouts, not instruments**: a fixed pool of
+cursors per row, created **unselectable** (`Plot::set_cursor_selectable()`, added for this), so
+the controller's hit test never offers the pointer one — it can neither hover nor drag a
+measurement's ruler. They are also re-placed every frame, because a mark is only correct for
+the window its measurement was computed over. Re-placing alone was not enough: a drag is held
+for the gesture's life, so the marker followed the pointer *between* frames and the readout
+looked draggable.
+
 ## The session layout file
 
 What you arranged is remembered per data source, in
@@ -409,8 +471,12 @@ figure so the legend name is right (there is one widget, so the row a drop lands
 on is `PlotArea::row_at()`). The status line reports the outcome of a drop
 (`plotting 'sine' on plot 1`, `plot 1 already holds four signals`), so an action
 that changes nothing still says why. Each plot's y axis has a right-click menu
-with auto scale, a fixed range, and one **Remove '<signal>' from plot N** entry per
-occupied trace slot (a plot with nothing on it says so instead).
+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.
 
 **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,
@@ -518,6 +584,55 @@ switches them on as well, because a reset nobody can see reads as a dead button.
 The two panels collapse vertically (`SLIDE_DOWN`), so hiding one gives its space
 back instead of leaving an invisible gap in the column.
 
+With the cursors on, a **readout panel** appears in the side column, and a dropdown
+chooses what the instruments are:
+
+| mode | instruments | the pane reads |
+|---|---|---|
+| `vertical` | two x markers, one pair for the whole figure | `T0`, `T1`, `dT` |
+| `horizontal` | two y rulers on **each** row | `Y1`, `Y2`, `dY` per row |
+| `both` | both of the above | the times, then a line per row |
+| `mix` | the x pair, plus where each marker **crosses each trace** | the times, then `Y1`, `Y2`, `dY` per plotted signal |
+
+The vertical pair is one stream time shared by every row, so the markers line up down
+the column and dragging one on any row moves them all — the app reads the position back
+out of the plot each frame, because a drag writes the *plot* and that is the only place
+it is visible. A ruler is per row: each row has its own y axis, so two rows' rulers are
+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.
+
+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
+rows below it are the rulers (one per row, in `horizontal`/`both`) or the traces (in
+`mix`). The time row is there only where the time cursors are: in `horizontal` the pair is
+parked, and a row of times nothing on the plot can move would be a readout of nothing. Columns size to their widest cell, so a column of numbers is a column; the two
+cursors are `A` and `B` and their difference is `delta`, whichever of the three the mode
+puts in them.
+
+Every readout is a **measurement**, not a raw number: at most two decimals, scaled by an
+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
+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** instead of its name — the chip identifies it exactly as the signal
+panel does — and the values are bare: `● 616.49 mV  -24.57 mV  -641.06 mV`, because the
+pane is a readout beside a plot and `Y1:` labels there are noise.
+
+`mix` also *draws* the crossings: a mark on each trace where each marker meets it, in the
+marker's own colour, placed with the same call the pane reads (so the mark and the number
+cannot disagree), and shown only where the trace actually has samples at that time. The
+marks are annotations the plot holds from construction and that are moved, never added:
+`Plot` appends annotations only, so a host that re-added them would grow the plot every
+frame — which is why the library gained `Plot::set_annotation()` for this. Nothing marks
+the ruler modes: a ruler *is* the mark.
+
 **The time window is typed, not picked off a ladder.** The header bar's `window`
 button reads the current span and opens onto `÷10` · a number · a unit (µs, ms, s,
 min) · `×10`. A ladder of round numbers could only ever offer the spans someone had
@@ -573,9 +688,11 @@ logged with both rates and the new anti-alias corner. A window change that leave
 `time_stride` alone rebuilds nothing. Per-channel `stride` is still always 1 (see the
 limitations below); only the shared clock is decimated.
 
-**Fit to data** (overflow menu) does what the plot's own `f` key does: auto-scales every row's y
-axis and re-frames the x window on the data the ring holds, propagated to every
-row. The status line acknowledges it (`fitted to the buffered data (1.25 s)`).
+**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)`).
 
 **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.
@@ -607,6 +724,11 @@ removes it, a `+` on each boundary inserts one there — so the row an action na
 row you are pointing at. `--plots` is now only the count to start with, and `RowSet::kMaxRows`
 (8) is a ceiling rather than a menu: past about that a row is chrome and no plot.
 
+The editor also carries its own way out: a **`Done`** button in the plot area's top-right
+corner while it is up, because the toggle that opened it lives in the overflow menu — a
+long way from the plot being edited. It turns that toggle off, so there is still exactly
+one piece of state, and the buttons (its own included) go with it.
+
 An insert or a remove **moves the rows' contents with them**: what row 1 plotted, in the
 colours it plotted it in and with its cursors where they were, is on row 2 after inserting
 above row 1. Only that state moves; the figure's `Plot`s are rebuilt and re-dealt, which is
@@ -711,6 +833,19 @@ now — they are listed so they are not mistaken for anything else:
   hundred samples every fifth of a second made every frame span microseconds while
   the frames were 100 ms apart).
 
+**One source, one client, over unicast.** A UDPStreamer over unicast keeps a single
+session: a second `udpscope`'s CONNECT displaces the first, the displaced client sees its
+`silence_timeout_s` elapse, reconnects and displaces the other, and the two take turns
+being served — which reads as "the second UI gets no data" (and both windows look alike
+because they share the saved layout). Nothing on the client side can give both of them the
+stream: the wire has one destination. Watching one source from several UIs is what
+`--multicast <group> [--interface <local ip>]` is for — every instance joins the group and
+the kernel hands each of them a copy. When the app sees the fight (a reconnect with no
+frames delivered in the same second — the client's own retry counter is also shown in the
+status line as `N reconnects` once it is nonzero) it says so in the status line rather than
+leaving an empty plot to be puzzled over, and prints a `starved:` line to stderr while the
+state holds.
+
 ## Limitations
 
 - **Multi-element (vector/matrix) signals are acquired with a zero-order hold.**
@@ -727,11 +862,22 @@ now — they are listed so they are not mistaken for anything else:
 - **Calibration is applied at ingest**, so changing gain or offset rebuilds the
   buffers (and drops the history). Applying it at read time would need the LOD
   pyramid to be gain-aware.
-- **Cursors are per row** and not synchronised with each other; show/hide parks
-  them outside the axis range rather than removing them, because `Plot` has no
-  `remove_cursor`. The trigger marker is the one exception: it has its own cursor
-  and is independent of the cursor toggle.
+- **The horizontal rulers are per row**, since a y value means nothing without the
+  y axis it is on; the vertical pair is shared, so the same two times are on every
+  row. Show/hide *parks* every instrument outside the axis range rather than removing
+  it, because `Plot` has no `remove_cursor` — which is why switching mode re-places
+  rather than rebuilds, and why the frame tick reads the markers back: a parked
+  cursor's snapped position is a real number, so nothing may be inferred from it. The
+  trigger marker is separate: its own cursor, independent of the cursor toggle.
 - **Verified headless only.** Acquisition, partitioning, the trigger state
   machine and its integration with the buffers, and rendering under Xvfb
   (llvmpipe) were exercised; the interactive paths (drag and drop, the dialogs,
   the right-click menu) were not driven by an automated test.
+
+**A debug-build assertion in the library's suite.** `deps/plotter`'s debug build prints
+`TextBatch::add_cell`'s `glyph_is_cell` assertion once while running its tests — from a
+forked child of the text/atlas cases (`a_backend_without_an_atlas_uploads_nothing` is the
+one that precedes it), not from a failing test: the suite still reports `355 cases,
+0 failed`, and this is pre-existing rather than something the cursors work introduced. The
+new ruler test asserts the *strokes* (`figure_rows_draw_their_own_horizontal_rulers`) with
+the harness's no-atlas backend, so it neither needs nor trips the text path.

src/plot_host.cpp

@@ -26,6 +26,10 @@ constexpr plot::u32 kUnplottedColor = 0x6E6E6Eu;
 constexpr plot::u32 kCursorAColor = 0xFFD24Fu;
 constexpr plot::u32 kCursorBColor = 0x7FE0FFu;
 constexpr plot::u32 kRulerColor = 0xB0B0B0u;
+constexpr plot::u32 kRulerBColor = 0x8FA8C8u;
+/// The measurements' own colour, distinct from both cursor rulers so a readout mark cannot be
+/// mistaken for something the user placed.
+constexpr plot::u32 kMeasureColor = 0x9BE8A0u;
 constexpr plot::u32 kTriggerColor = 0xFF5B5Bu;
 
 constexpr double kParked = -1.0e12;  ///< hides an instrument without removing it
@@ -59,10 +63,9 @@ void RowSet::carry_row(Row& dst, const Row& src) {
   dst.name = src.name;
   dst.channel = src.channel;
   dst.color_index = src.color_index;
-  dst.cursors_visible = src.cursors_visible;
-  dst.cursor_a_x = src.cursor_a_x;
-  dst.cursor_b_x = src.cursor_b_x;
+
   dst.ruler_y = src.ruler_y;
+  dst.ruler_b_y = src.ruler_b_y;
   // The slot's series stays where it is, but what it is bound to belonged to the row
   // this state came *from*. `bind()` re-resolves every slot next frame; clearing it here
   // is what stops the frame in between from drawing the moved row's data under the new
@@ -75,9 +78,6 @@ void RowSet::clear_row(Row& r) {
   r.name = {};
   r.channel = {-1, -1, -1, -1};
   r.color_index = {0, 1, 2, 3};
-  r.cursors_visible = false;
-  r.cursor_a_x = 0.0;
-  r.cursor_b_x = 0.0;
   r.ruler_y = 0.0;
   for (SlotSeries& s : r.slots) s.bind(nullptr);
 }
@@ -115,6 +115,30 @@ void RowSet::install(Row& r, int /*row_index*/, plot::Plot& p) {
   r.cursor_a = r.plot->add_cursor(0.0, plot::Color::hex(kCursorAColor));
   r.cursor_b = r.plot->add_cursor(0.0, plot::Color::hex(kCursorBColor));
   r.ruler = r.plot->add_h_ruler(0.0, plot::Color::hex(kRulerColor));
+  r.ruler_b = r.plot->add_h_ruler(0.0, plot::Color::hex(kRulerBColor));
+  // The `mix` crossings, one pair per trace slot, hidden until that mode is on. Created
+  // here because a `Plot` fixes its annotation set the same way it fixes its traces --
+  // `add_annotation()` only appends -- so the markers are made up front and moved.
+  for (int s = 0; s < kSlotsPerRow; ++s) {
+    plot::Annotation a{0.0, 0.0, "", plot::Color::hex(kCursorAColor), r.axis_x, r.axis_y, false};
+    plot::Annotation b{0.0, 0.0, "", plot::Color::hex(kCursorBColor), r.axis_x, r.axis_y, false};
+    r.crossing_a[static_cast<std::size_t>(s)] = r.plot->add_annotation(a);
+    r.crossing_b[static_cast<std::size_t>(s)] = r.plot->add_annotation(b);
+  }
+  // The measurements' readout marks: a fixed pool, parked. Levels are rulers (they read a y)
+  // and times are markers (they read an x), so the pool has one of each per slot.
+  for (int m = 0; m < kMeasureMarks; ++m) {
+    const std::size_t mi = static_cast<std::size_t>(m);
+    r.measure_h[mi] = r.plot->add_h_ruler(kParked, plot::Color::hex(kMeasureColor));
+    r.measure_v[mi] = r.plot->add_cursor(kParked, plot::Color::hex(kMeasureColor));
+    // Readouts, not instruments: the pointer must not be able to hover or grab one. Parking
+    // them outside the axis and re-placing every frame does not stop a *drag* -- the
+    // controller holds the marker for the gesture's life, so it follows the pointer between
+    // frames and the readout looks draggable. Excluding it from the hit test is the only
+    // place that can say no.
+    r.plot->set_cursor_selectable(r.measure_h[mi], false);
+    r.plot->set_cursor_selectable(r.measure_v[mi], false);
+  }
   r.trigger_marker = r.plot->add_cursor(kParked, plot::Color::hex(kTriggerColor));
   // A marker, not an instrument: it must sit exactly on the edge's time, so its
   // snap is Free and the controller never rounds it onto a sample.
@@ -303,13 +327,190 @@ void RowSet::set_y_range(int row_index, double lo, double hi) {
 
 void RowSet::place_cursors(Row& r) {
   if (!r.plot) return;
-  const bool show = r.cursors_visible;
-  const double a = show ? r.cursor_a_x : kParked;
-  const double b = show ? r.cursor_b_x : kParked;
-  const double h = show ? r.ruler_y : kParked;
-  r.plot->set_cursor_x(r.cursor_a, a);
-  r.plot->set_cursor_x(r.cursor_b, b);
-  r.plot->set_cursor_y(r.ruler, h);
+  const bool on = cursors_on_;
+  // Which instruments the mode uses; the rest are parked (a `Plot` cannot drop a
+  // cursor, so hiding is the only way to take one off screen).
+  const bool vertical = on && cursor_mode_ != CursorMode::Horizontal;
+  const bool horizontal = on && (cursor_mode_ == CursorMode::Horizontal ||
+                                 cursor_mode_ == CursorMode::Both);
+  r.plot->set_cursor_x(r.cursor_a, vertical ? cursor_a_x_ : kParked);
+  r.plot->set_cursor_x(r.cursor_b, vertical ? cursor_b_x_ : kParked);
+  r.plot->set_cursor_y(r.ruler, horizontal ? r.ruler_y : kParked);
+  r.plot->set_cursor_y(r.ruler_b, horizontal ? r.ruler_b_y : kParked);
+}
+
+void RowSet::set_cursor_mode(CursorMode mode) {
+  if (mode == cursor_mode_) return;
+  cursor_mode_ = mode;
+  for (int i = 0; i < kMaxRows; ++i) place_cursors(row(i));
+  place_crossings();
+}
+
+void RowSet::place_crossings() {
+  const bool mix = cursor_mode_ == CursorMode::Mix && cursors_visible();
+  for (int i = 0; i < kMaxRows; ++i) {
+    Row& r = row(i);
+    if (!r.plot) continue;
+    for (int s = 0; s < kSlotsPerRow; ++s) {
+      const std::size_t si = static_cast<std::size_t>(s);
+      const plot::u32 ida = r.crossing_a[si];
+      const plot::u32 idb = r.crossing_b[si];
+      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 ya = 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);
+      a.visible = has_a;
+      b.visible = has_b;
+      if (has_a) {
+        a.x = cursor_a_x_;
+        a.y = ya;
+      }
+      if (has_b) {
+        b.x = cursor_b_x_;
+        b.y = yb;
+      }
+      r.plot->set_annotation(ida, a);
+      r.plot->set_annotation(idb, b);
+    }
+  }
+}
+
+double RowSet::ruler_y(int r) const {
+  return r >= 0 && r < rows_ ? row(r).ruler_y : 0.0;
+}
+
+double RowSet::ruler_b_y(int r) const {
+  return r >= 0 && r < rows_ ? row(r).ruler_b_y : 0.0;
+}
+
+int RowSet::crossings_visible() const {
+  int n = 0;
+  for (int i = 0; i < rows_; ++i) {
+    const Row& r = row(i);
+    if (!r.plot) continue;
+    for (int s = 0; s < kSlotsPerRow; ++s) {
+      const std::size_t si = static_cast<std::size_t>(s);
+      if (r.crossing_a[si] < r.plot->annotation_count() &&
+          r.plot->annotation(r.crossing_a[si]).visible) {
+        ++n;
+      }
+      if (r.crossing_b[si] < r.plot->annotation_count() &&
+          r.plot->annotation(r.crossing_b[si]).visible) {
+        ++n;
+      }
+    }
+  }
+  return n;
+}
+
+bool RowSet::crossing_at(int i, int slot, bool second, double& x, double& y) const {
+  if (i < 0 || i >= rows_ || slot < 0 || slot >= kSlotsPerRow) return false;
+  const Row& r = row(i);
+  if (!r.plot) return false;
+  const std::size_t si = static_cast<std::size_t>(slot);
+  const plot::u32 id = second ? r.crossing_b[si] : r.crossing_a[si];
+  if (id >= r.plot->annotation_count()) return false;
+  const plot::Annotation& a = r.plot->annotation(id);
+  if (!a.visible) return false;
+  x = a.x;
+  y = a.y;
+  return true;
+}
+
+void RowSet::set_measure_marks(const std::vector<MeasureMark>& marks) {
+  measure_marks_placed_ = static_cast<int>(marks.size());
+  for (int i = 0; i < kMaxRows; ++i) {
+    Row& r = row(i);
+    if (!r.plot) continue;
+    int h = 0;
+    int v = 0;
+    for (const MeasureMark& m : marks) {
+      if (m.row != i) continue;
+      if (m.level) {
+        if (h >= kMeasureMarks) continue;
+        const std::size_t mi = static_cast<std::size_t>(h++);
+        r.plot->set_cursor_y(r.measure_h[mi], m.value, plot::CursorPlacement::Free);
+        r.plot->set_cursor_color(r.measure_h[mi], plot::Color::hex(m.color));
+      } else {
+        if (v >= kMeasureMarks) continue;
+        const std::size_t mi = static_cast<std::size_t>(v++);
+        r.plot->set_cursor_x(r.measure_v[mi], m.value, plot::CursorPlacement::Free);
+        r.plot->set_cursor_color(r.measure_v[mi], plot::Color::hex(m.color));
+      }
+    }
+    // What the measurements left over is parked rather than kept: a stale readout at the old
+    // window's position is worse than none.
+    for (; h < kMeasureMarks; ++h) {
+      r.plot->set_cursor_y(r.measure_h[static_cast<std::size_t>(h)], kParked);
+    }
+    for (; v < kMeasureMarks; ++v) {
+      r.plot->set_cursor_x(r.measure_v[static_cast<std::size_t>(v)], kParked);
+    }
+  }
+}
+
+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);
+  }
+}
+
+void RowSet::sync_cursors() {
+  if (!cursors_visible()) return;
+  // Only a mode that *shows* the vertical pair reads them back. With them parked (the
+  // `horizontal` mode) whatever the plot reports is our own park coming back -- and a
+  // snapped cursor parked far outside the axis reads back as something finite, which
+  // would be adopted as a gesture and destroy the shared times.
+  // Anything within this much of what we last pushed is our own placement coming back
+  // (the plot stores f64, so an exact compare would be brittle across a rebuild).
+  const auto moved = [](double a, double b) {
+    return std::fabs(a - b) > 1.0e-9 * std::max(1.0, std::fabs(b));
+  };
+  bool push = false;
+  for (int i = 0; i < rows_; ++i) {
+    Row& r = row(i);
+    if (!r.plot) continue;
+    // A gesture writes the *plot*; this is the only place its result is visible. A
+    // ruler is its own row's value -- nothing to share, but it still has to be read
+    // back or the pane would keep showing wherever the app last put it.
+    const bool rulers = cursor_mode_ == CursorMode::Horizontal || cursor_mode_ == CursorMode::Both;
+    if (rulers) {
+      const double y = r.plot->cursor(r.ruler).y;
+      const double yb = r.plot->cursor(r.ruler_b).y;
+      if (y > kParked * 0.5) r.ruler_y = y;
+      if (yb > kParked * 0.5) r.ruler_b_y = yb;
+    }
+    if (cursor_mode_ == CursorMode::Horizontal) continue;  // the pair is parked: not a value
+    const double a = r.plot->cursor(r.cursor_a).x;
+    const double b = r.plot->cursor(r.cursor_b).x;
+    if (a < kParked * 0.5 || b < kParked * 0.5) continue;  // parked: not a value
+    if (moved(a, cursor_a_x_)) {
+      cursor_a_x_ = a;
+      push = true;
+    }
+    if (moved(b, cursor_b_x_)) {
+      cursor_b_x_ = b;
+      push = true;
+    }
+  }
+  if (push) {
+    for (int i = 0; i < rows_; ++i) place_cursors(row(i));
+  }
+  place_crossings();
 }
 
 void RowSet::clamp_to_data() {
@@ -442,11 +643,8 @@ void RowSet::set_trigger_marker(double x, bool visible) {
 }
 
 void RowSet::set_cursors_visible(bool on) {
-  for (int i = 0; i < kMaxRows; ++i) {
-    Row& r = row(i);
-    r.cursors_visible = on;
-    place_cursors(r);
-  }
+  cursors_on_ = on;
+  for (int i = 0; i < kMaxRows; ++i) place_cursors(row(i));
 }
 
 void RowSet::reset_cursors() {
@@ -455,13 +653,18 @@ void RowSet::reset_cursors() {
     if (!r.plot) continue;
     const plot::Range xr = r.plot->x_range(r.axis_x);
     const double span = xr.span() > 0.0 ? xr.span() : window_s_;
-    r.cursor_a_x = xr.min + 0.25 * span;
-    r.cursor_b_x = xr.min + 0.75 * span;
-    r.ruler_y = r.plot->y_range(r.axis_y).min +
-                0.5 * (r.plot->y_range(r.axis_y).span() > 0.0 ? r.plot->y_range(r.axis_y).span()
-                                                              : 1.0);
-    place_cursors(r);
+    cursor_a_x_ = xr.min + 0.25 * span;
+    cursor_b_x_ = xr.min + 0.75 * span;
+    const double y_lo = r.plot->y_range(r.axis_y).min;
+    const double y_span = r.plot->y_range(r.axis_y).span();
+    r.ruler_y = y_lo + 0.35 * (y_span > 0.0 ? y_span : 1.0);
+    r.ruler_b_y = y_lo + 0.65 * (y_span > 0.0 ? y_span : 1.0);
   }
+  // Every row's instruments are written from what was just set. Without this the plots
+  // keep their build-time cursors (x = 0) and `sync_cursors()` -- which reads the plots
+  // because a *gesture* writes them -- adopts that zero as a moved cursor.
+  for (int i = 0; i < kMaxRows; ++i) place_cursors(row(i));
+  place_crossings();
 }
 
 }  // namespace udpscope

src/plot_host.hpp

@@ -48,9 +48,27 @@ class RowSet {
   /// about this many a row is chrome and no plot.
   static constexpr int kMaxRows = 8;
   static constexpr int kSlotsPerRow = 4;
+  /// Readout marks per row per orientation: enough for a handful of level and time
+  /// measurements to be shown at once, and a fixed number because a `Plot` fixes its cursor
+  /// set at construction.
+  static constexpr int kMeasureMarks = 6;
 
   RowSet();
 
+  /// Which cursor instruments are up, from the toolbar's toggle. The two vertical
+  /// markers are shared by every row (one stream time each, so they line up down the
+  /// column); a horizontal ruler is per row, because each row has its own y axis.
+  ///
+  /// `Mix` is the vertical pair with the *per-trace* readout instead of rulers: the
+  /// pane then reports where each cursor crosses each trace, which is the value a
+  /// ruler would have to be dragged to express.
+  enum class CursorMode {
+    Vertical,    ///< two x markers, shared by all rows
+    Horizontal,  ///< two y rulers per row
+    Both,        ///< both instruments
+    Mix,         ///< the x pair, read off each trace
+  };
+
   struct Row {
     /// The figure's row, borrowed. Null for a row beyond `row_count()`, which is how
     /// every loop here and in the UI stays inside the rows that exist. Invalidated by
@@ -82,16 +100,28 @@ class RowSet {
     plot::u32 cursor_a = 0;
     plot::u32 cursor_b = 0;
     plot::u32 ruler = 0;
+    plot::u32 ruler_b = 0;
     /// The vertical line marking the trigger instant, drawn whenever an event is
     /// on screen (and parked outside the axis otherwise). Not the user's cursor:
     /// it keeps its position whatever the cursor toggle says.
     plot::u32 trigger_marker = 0;
-    /// Off by default: the cursors are switched on from the toolbar, and a rebuild
-    /// (any drop, any row-count change) must not switch them back off.
-    bool cursors_visible = false;
-    double cursor_a_x = 0.0;
-    double cursor_b_x = 0.0;
+
+    /// The measurements' readout marks: a pool created with the plot (a `Plot` cannot add or
+    /// drop a cursor after the fact) and parked unless a measurement is using the slot.
+    std::array<plot::u32, kMeasureMarks> measure_h{};
+    std::array<plot::u32, kMeasureMarks> measure_v{};
+
+    /// One marker per trace per cursor: where each vertical cursor crosses that trace's
+    /// series, drawn only in `mix` mode. Two per slot because the pair crosses a trace
+    /// at two places, and an annotation cannot be shared between them.
+    std::array<plot::u32, kSlotsPerRow> crossing_a{};
+    std::array<plot::u32, kSlotsPerRow> crossing_b{};
+
+    /// The two rulers' values. The vertical pair is *not* here: one stream time is
+    /// shared by every row, so it lives on the `RowSet` where it cannot drift between
+    /// rows.
     double ruler_y = 0.0;
+    double ruler_b_y = 0.0;
   };
 
   int row_count() const noexcept { return rows_; }
@@ -152,6 +182,67 @@ class RowSet {
   void set_y_range(int row, double lo, double hi);
 
   void set_cursors_visible(bool on);
+  /// What the cursors are (see `CursorMode`). Switching parks the instruments the mode
+  /// does not use rather than removing them: a `Plot` cannot drop a cursor.
+  void set_cursor_mode(CursorMode mode);
+  /// Places (or hides) the `mix` mode's crossing markers: for every plotted trace, the
+  /// point where each cursor meets it. Called from `sync_cursors()` and when the mode
+  /// changes, so the markers are never a frame behind the markers that cast them.
+  void place_crossings();
+  CursorMode cursor_mode() const noexcept { return cursor_mode_; }
+
+  /// The two vertical cursors' stream times; shared by every row.
+  double cursor_a_x() const noexcept { return cursor_a_x_; }
+  double cursor_b_x() const noexcept { return cursor_b_x_; }
+  /// One row's ruler values, or 0 for a row that does not exist.
+  double ruler_y(int row) const;
+  double ruler_b_y(int row) const;
+  /// How many `mix` crossing markers are on screen, and the first one's position: the
+  /// probe's window onto "the marks follow the cursors".
+  int crossings_visible() const;
+  bool crossing_at(int row, int slot, bool second, double& x, double& y) const;
+
+  /// Whether the cursors are on. One flag for the whole set, not a copy per row: it is one
+  /// piece of state (the toolbar's toggle), and a per-row copy is something a rebuild, an
+  /// insert or a carried row can clear behind the user's back -- which is how a row could
+  /// end up with its instruments parked while every other row showed them.
+  bool cursors_visible() const noexcept { return cursors_on_; }
+
+  /// One measurement's readout mark on a row: a *level* (drawn as a horizontal ruler) or a
+  /// *time* (a vertical marker). Which one a measurement needs is the measurement's own
+  /// business; the row is the one that plots its signal.
+  struct MeasureMark {
+    int row = 0;
+    bool level = false;
+    double value = 0.0;
+    plot::u32 color = 0;
+  };
+
+  /// Places the measurements' marks and parks every pool slot they do not use. The marks are
+  /// created unselectable (`Plot::set_cursor_selectable()`), so the pointer can neither hover
+  /// nor drag them; this runs every frame because a mark is only correct for the window its
+  /// measurement was computed over.
+  void set_measure_marks(const std::vector<MeasureMark>& marks);
+  /// How many marks the last `set_measure_marks()` placed: the probe's window onto "the
+  /// measurement is drawn", as opposed to merely computed.
+  int measure_marks_placed() const noexcept { return measure_marks_placed_; }
+
+  /// 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.
+  void fit_x();
+
+  /// Adopts the instruments a gesture moved. The vertical pair is one time for the whole
+  /// figure, so a drag on any row's marker moves every row's; a ruler belongs to its own
+  /// row and is only read back. Either way the plot is where a drag writes, so this is the
+  /// one place the app can see it -- and the pane reads what is stored here, which is why a
+  /// ruler that was not read back showed a stale value while its marker on the plot moved.
+  /// Called once a frame from the tick.
+  void sync_cursors();
+
+  /// Puts both vertical cursors at a quarter and three quarters of the x window, and
+  /// the two rulers at a third and two thirds of each row's y range. Also switches the
+  /// cursors on, so the button cannot look dead.
   void reset_cursors();
 
   /// Places the trigger marker at stream time `x` (seconds since the epoch), or
@@ -237,6 +328,12 @@ class RowSet {
   double y_gutter_px_ = 0.0;
   bool y_gutter_reserved_ = false;
   std::array<Row, static_cast<std::size_t>(kMaxRows)> storage_{};
+  CursorMode cursor_mode_ = CursorMode::Vertical;
+  bool cursors_on_ = false;
+  int measure_marks_placed_ = 0;
+  /// The one stream time each vertical cursor is at, whatever row's copy was last moved.
+  double cursor_a_x_ = 0.0;
+  double cursor_b_x_ = 0.0;
   std::array<bool, static_cast<std::size_t>(kMaxRows)> row_dirty_{};
 };