fix: the t0 caption is chrome anchored to the axis, not an annotation in the data

The caption that names the relative x labels' anchor was an `Annotation`, whose anchor
is a data coordinate -- so it crawled across the plot under a pan and drifted under a
zoom, which is how it was reported. It is now `Plot::set_x_note()`: the bottom row's
caption, drawn by the renderer in the axes' bottom-left corner in pixels, on the
legend's own backplate.

The text stays the host's, because only the host knows that label + caption has to be
the exact absolute time (`%.6g`, where `format_time()` would round to three figures).

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

CLAUDE.md

@@ -1437,6 +1437,21 @@ far the largest file.
   is exactly enough), so the bare comparison picked a different form from one frame to the next.
   The guard is `[timelabels-probe]`'s `distinct=1`, which asserts what the form is *for* rather
   than which form a given window gets.
+- **The `t0 = …` caption is chrome, never an annotation.** It is `Plot::set_x_note()` (added to
+  `deps/plotter` for this), drawn by `Renderer::draw_x_note()` on the legend's own
+  `draw_readout_panel()` backplate in the bottom-left corner of the axes, in **pixels**. It was an
+  `Annotation` first, and an annotation's anchor is a *data* coordinate: the caption therefore
+  crawled across the plot under a pan and drifted under a zoom, which is what "[t0=...s] move
+  slightly around when user zoom/pan" was. Three things follow. The **text is the host's**, because
+  the caption has to name the anchor precisely enough that label + caption is the exact absolute
+  time and `format_time()` would round it to three figures (`"t0 = %.6g s"`); only the **bottom
+  row** of the figure carries one, every other row being set to `""`; and the renderer measures it
+  rather than `Plot::layout()` — it reserves no space and nothing can be picked on it, so there is
+  no rect to read back, and a caption explaining the x labels must not be able to move the layout
+  those labels were measured into. The guards are the library's
+  `the_x_note_is_chrome_that_a_pan_or_a_zoom_does_not_move` (the footprint is the pixels a frame
+  with the caption differs from one without, and a pan plus a 64× zoom must give the same box to
+  the pixel) and `[timelabels-probe]`'s `note=`, which reads `Plot::x_note()` back.
 - **`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

README.md

@@ -95,9 +95,15 @@ feedback loop), the magnitude is the larger of the two endpoints, and the compar
 1.05 band leaving the relative form plus a ULP of slack entering it — a step *equal* to the
 resolution is readable (`1.73s`, `1.74s`, `1.75s`: one in the last digit is exactly enough), so a
 decision taken on the last bits of two differently computed doubles picked a different form from
-one frame to the next. The caption is an annotation
-inside the axes rather than the axis title on purpose: its text changes with the window, and a
-title of a changing width would relayout the plot every frame. Everything a user copies out —
+one frame to the next. The caption is **chrome** — `Plot::set_x_note()`, added to the plotter for
+it, drawn on the legend's own backplate in the bottom-left corner of the axes — rather than the
+axis title or an annotation. Not the title, because its text changes with the window and a title
+of a changing width would relayout the plot every frame; not an annotation, because an
+annotation's anchor is a *data* coordinate, so the caption crawled across the plot under a pan and
+drifted under a zoom, which is what it was reported as. The library test is
+`the_x_note_is_chrome_that_a_pan_or_a_zoom_does_not_move`: the footprint is measured as the pixels
+a frame with the caption differs from the same frame without one, and a pan to another decade plus
+a 64× zoom must produce the same box to the pixel. Everything a user copies out —
 the cursors' `T0`/`T1`/`dT`, a measurement, an export — stays in absolute stream seconds.
 
 The anchor is taken by the *axis* on every layout (`Axis::origin_follows_range`), not written

src/plot_host.cpp

@@ -30,8 +30,6 @@ 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;
-/// The window-start caption's colour: quiet chrome, brighter than a grid line.
-constexpr plot::u32 kTimeNoteColor = 0xC8C8C8u;
 constexpr plot::u32 kTriggerColor = 0xFF5B5Bu;
 
 /// The smallest difference a `TickFormat::Time` label can *write* at the magnitude `t` -- what
@@ -170,11 +168,6 @@ void RowSet::install(Row& r, int /*row_index*/, plot::Plot& p) {
     r.plot->set_cursor_stroke(r.measure_v[mi], kMeasureStrokeWidth,
                               plot::Vec2{kMeasureDashOn, kMeasureDashOff});
   }
-  // The window-start caption for the relative x labels: an annotation, hidden until the
-  // labels are relative. Inside the axes on purpose -- its text changes with the window, and
-  // an axis title of a changing width would relayout the plot every frame.
-  r.time_origin_note = r.plot->add_annotation(
-      plot::Annotation{0.0, 0.0, "", plot::Color::hex(kTimeNoteColor), r.axis_x, r.axis_y, 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.
@@ -581,27 +574,25 @@ bool RowSet::update_time_labels() {
     r.plot->set_x_axis_origin_from_range(r.axis_x, relative_x_);
     if (!relative_x_) r.plot->set_x_axis_origin(r.axis_x, 0.0);
     // The caption belongs with the labels, which a figure draws once, on the bottom row (the
-    // one whose x axis is the figure's). Every row gets it parked.
+    // one whose x axis is the figure's). Every other row gets an empty one, which draws nothing.
     const bool bottom = i == rows_ - 1;
-    if (r.time_origin_note >= r.plot->annotation_count()) continue;
-    plot::Annotation note = r.plot->annotation(r.time_origin_note);
-    note.visible = relative_x_ && bottom;
-    if (note.visible) {
-      // Inside the axes, just above the axis line, at their left edge: the reader adds this
-      // to a tick, so it sits beside the ticks.
-      const plot::Range yr = r.plot->y_range(r.axis_y);
-      note.x = view.min;
-      note.y = yr.min + 0.04 * (yr.span() > 0.0 ? yr.span() : 1.0);
-      char text[64];
-      // The anchor the *drawn* labels were built with: the axis' own origin, which the last
-      // layout set from its range. Reading it back (rather than keeping our own copy) is what
-      // makes the reader's addition exact -- label plus caption is that tick's absolute time.
-      const double shown = r.plot->x_axis_origin(r.axis_x);
-      x_origin_ = shown;
-      std::snprintf(text, sizeof(text), "t0 = %.6g s", shown);
-      note.text = text;
+    if (!relative_x_ || !bottom) {
+      r.plot->set_x_note("");
+      continue;
     }
-    r.plot->set_annotation(r.time_origin_note, note);
+    char text[64];
+    // The anchor the *drawn* labels were built with: the axis' own origin, which the last
+    // layout set from its range. Reading it back (rather than keeping our own copy) is what
+    // makes the reader's addition exact -- label plus caption is that tick's absolute time.
+    const double shown = r.plot->x_axis_origin(r.axis_x);
+    x_origin_ = shown;
+    std::snprintf(text, sizeof(text), "t0 = %.6g s", shown);
+    // Chrome, not an annotation: an annotation is anchored to a *data* coordinate, so the caption
+    // crawled across the plot as the view panned and drifted as it zoomed -- and it came with the
+    // annotation's own pin cross, which pointed at nothing. `set_x_note()` places it in the axes'
+    // bottom-left corner in pixels, on the legend's backplate, which is where the labels it
+    // qualifies are.
+    r.plot->set_x_note(text);
   }
   return changed;
 }

src/plot_host.hpp

@@ -106,11 +106,6 @@ class RowSet {
     /// it keeps its position whatever the cursor toggle says.
     plot::u32 trigger_marker = 0;
 
-    /// The caption naming the window's start while the x labels are relative: an annotation
-    /// inside the axes, so its text can change every frame without moving the layout (an axis
-    /// *title* would, and it would be a different width each time).
-    plot::u32 time_origin_note = 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{};

src/ui.cpp

@@ -7264,10 +7264,7 @@ gboolean timelabels_probe_step(gpointer user) {
         ticks += t.label;
       }
     }
-    if (last.time_origin_note < last.plot->annotation_count()) {
-      const plot::Annotation& an = last.plot->annotation(last.time_origin_note);
-      if (an.visible) note = an.text;
-    }
+    note = last.plot->x_note();
   }
   double first_value = 0.0;
   plot::Range shown{};
@@ -8317,14 +8314,12 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
     }
     {
       // The x label form: relative to the window's own start (with the caption that names
-      // that start) while the span is sub-second, absolute otherwise. `x_origin_set` is read
-      // back out of the plot, so this is what the axis is actually formatting with.
+      // that start) when the labels could not carry the tick step absolutely, absolute
+      // otherwise. `x_origin_set` is read back out of the plot, so this is what the axis is
+      // actually formatting with, and `note` is the plot's own caption string.
       const RowSet::Row& rlt = rows.row(rows.row_count() - 1);
       std::string note;
-      if (rlt.plot != nullptr && rlt.time_origin_note < rlt.plot->annotation_count()) {
-        const plot::Annotation& a = rlt.plot->annotation(rlt.time_origin_note);
-        if (a.visible) note = a.text;
-      }
+      if (rlt.plot != nullptr) note = rlt.plot->x_note();
       // The labels themselves, from the axis the reader looks at: the assertion is what the
       // ticks *say*, not only which mode is on.
       std::string ticks;