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>

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;
 }