crosshair: axis badges, clustered cursor labels, and the shared-y horizontal line

- Axis badges: the y value in the y label strip and the x time in the x strip, each a
  full-opacity panel in the crosshair's colour, **centred on the line it names**. The tick
  text is flushed before the fill -- the over-text batch is flushed after every line batch,
  so without that flush the label behind printed through the panel (the "transparent
  backplate" defect).
- Cursor labels: collected across the cursor loop and clustered at its end, so two marks in
  one column share a panel with one line each instead of overprinting, and a label stays on
  its own mark's row.
- Figure: a split row's cells share one y (`set_share_row_y`), so the horizontal line is
  drawn in every cell of the hovered row -- one value, one scale, spanning the row -- and
  still in no other row.

Tests: `figure_crosshair_horizontal_spans_a_split_rows_cells`, plus the render-smoke
additions for the badge placement/flush and the clustered labels.

src/plot/figure.cpp

@@ -819,6 +819,21 @@ void Figure::set_crosshair(f64 x, u32 hovered_row, f64 y_in_row, bool active) {
   // than clearing a hover the pointer still has.
   if (!active || hovered_row >= plots_.size()) return;
   plots_[hovered_row].set_crosshair(x, y_in_row, true);
+  // **The cells of a split row share one y**, so the horizontal line means the same value in
+  // every one of them: it spans the row rather than stopping at the divider between two cells
+  // that read the same scale. Only that row -- rows do not share y with each other, so a
+  // sibling row's line would be a value on a scale it does not have -- and only when the row
+  // really is split (`set_share_row_y`, which is what makes the y common in the first place).
+  if (!share_row_y_) return;
+  const u32 row = row_of_plot(hovered_row);
+  if (row >= cells_per_row_.size() || cells_per_row_[row] <= 1u) return;
+  const u32 first = row_first_[row];
+  for (u32 k = 0; k < cells_per_row_[row]; ++k) {
+    const u32 cell = first + k;
+    if (cell != hovered_row && cell < plots_.size()) {
+      plots_[cell].set_crosshair(x, y_in_row, true);
+    }
+  }
 }
 
 void Figure::clear_crosshair() {

src/plot/plot.cpp

@@ -311,6 +311,11 @@ void Plot::set_cursor_color(u32 id, Color color) {
   cursors_[id].color = color;
 }
 
+void Plot::set_cursor_label(u32 id, std::string label) {
+  if (id >= cursors_.size()) return;
+  cursors_[id].label = std::move(label);
+}
+
 void Plot::set_cursor_stroke(u32 id, f32 width, Vec2 dash) {
   if (id >= cursors_.size()) return;
   cursors_[id].width = width;
@@ -1888,6 +1893,14 @@ void Plot::build_crosshair() {
   const auto inside = [](f32 v, f32 lo, f32 hi) {
     return std::clamp(v, std::min(lo, hi), std::max(lo, hi));
   };
+  crosshair.y_axis_badge = Rect{};
+  crosshair.y_axis_text.clear();
+  crosshair.x_axis_badge = Rect{};
+  crosshair.x_axis_text.clear();
+  if (!crosshair_.active) {
+    crosshair.rows.clear();
+    return;
+  }
 
   // The two lines, each a stroke box held *inside* the axes by half its own width. The
   // renderer stages the stroke across the box's centre line, so this is the whole placement
@@ -1918,6 +1931,58 @@ void Plot::build_crosshair() {
     crosshair.horizontal = Rect{axes.x, at - half, axes.w, stroke};
   }
 
+  // The axis badges, when asked for (`Crosshair::axis_labels`): the values the two lines
+  // meet the axes at, written in the strips the tick labels are written in and in each
+  // axis' own format, so a badge reads exactly like the tick beside it. Placed here rather
+  // than by the renderer, for the readout panel's reason -- one placement rule, one place to
+  // read it -- and left empty whenever there is nothing honest to place: no text backend to
+  // measure with, the line they belong to not drawn (a vertical-only crosshair has no y),
+  // or their strip absent or too small (a row whose x strip went to the row below, a y
+  // gutter collapsed to a title). The x badge is held to the axes' own columns as well, so
+  // a crosshair in the leftmost cell of a split row cannot badge into its neighbour's.
+  if (crosshair_.axis_labels && text_ != nullptr) {
+    const f32 badge_pad = style_.device(4.0f);
+    const f32 badge_h = text_->height() + 2.0f * badge_pad;
+    if (crosshair_.horizontal && layout_.y_label_area.w > 0.0f &&
+        layout_.y_label_area.h >= badge_h) {
+      char text[64];
+      y_axis(0u).format_value(crosshair_.y, text, sizeof text);
+      crosshair.y_axis_text.assign(text);
+      const f32 w = text_->measure(crosshair.y_axis_text.c_str()) + 2.0f * badge_pad;
+      // Right-aligned on the axis line -- where the y ticks end -- and **centred** on the
+      // horizontal line's own height, the way the x badge is centred on its column: the badge
+      // names the line it sits beside, and a badge whose top edge *is* that line reads as
+      // belonging to the tick below it. A badge is as wide as its own text and is capped by
+      // nothing else, so it
+      // is held within the **left strips' union**: the label strip first, the title strip
+      // beside it when there is one, and never over the data or off the viewport's left
+      // edge. A `std::clamp` on an inverted pair is undefined, which is exactly the state
+      // "the strip is narrower than the badge" produces -- hence the ordered min/max chain.
+      const f32 left = layout_.y_title_area.w > 0.0f ? layout_.y_title_area.x
+                                                     : layout_.y_label_area.x;
+      f32 bx = layout_.y_label_area.right() - w;
+      if (bx < left) bx = left;
+      bx = std::min(bx, axes.x - w);
+      if (bx < left) bx = left;  // a badge wider than the union stays at its left edge
+      const f32 at = inside(static_cast<f32>(ty.to_screen_y(crosshair_.y)) - 0.5f * badge_h,
+                            layout_.y_label_area.y + badge_pad,
+                            layout_.y_label_area.bottom() - badge_pad - badge_h);
+      crosshair.y_axis_badge = Rect{bx, at, w, badge_h};
+    }
+    if (crosshair_.vertical && layout_.x_label_area.w > 0.0f &&
+        layout_.x_label_area.h >= badge_h) {
+      char text[64];
+      x_axis(0u).format_value(crosshair_.x, text, sizeof text);
+      crosshair.x_axis_text.assign(text);
+      const f32 w = text_->measure(crosshair.x_axis_text.c_str()) + 2.0f * badge_pad;
+      const f32 column =
+          inside(static_cast<f32>(tx.to_screen_x(crosshair_.x)), axes.x, axes.right());
+      const f32 x = inside(column - 0.5f * w, layout_.x_label_area.x,
+                           layout_.x_label_area.right() - w);
+      crosshair.x_axis_badge = Rect{x, layout_.x_label_area.y, w, badge_h};
+    }
+  }
+
   // The rows: one per *visible* trace -- a trace with no series or no samples is not under
   // the pointer, so it has nothing to say -- named or not, because the readout is about what
   // the pointer is over and not about identity. The vector is filled in place and its entries

src/render/renderer.cpp

@@ -565,6 +565,10 @@ void Renderer::draw_plot(Plot& plot, const PlotLayout& layout) {
   clear_clip();
   draw_tick_labels(plot, layout);
   draw_titles(plot, layout);
+  // The crosshair's axis badges, last of all: they sit in the label strips among the tick
+  // labels, so they are drawn after them and unclipped -- and over them, which is the point
+  // (a badge the ticks print through is two numbers in one place).
+  draw_crosshair_axis_badges(layout);
 }
 
 void Renderer::end_frame() noexcept {
@@ -1675,6 +1679,9 @@ void Renderer::draw_cursors(const Plot& plot, const PlotLayout& layout) {
   // does not depend on the marker's own colour differing from the accent -- a cursor
   // created in the accent colour is otherwise indistinguishable from an idle one.
   const HotHandles hot = plot.hot_handles();
+  // Collected across the loop below and drawn clustered at its end -- see
+  // `draw_clustered_cursor_labels()` for why one panel can carry several lines.
+  std::vector<CursorLabel> labels;
 
   for (u32 i = 0; i < plot.cursor_count(); ++i) {
     const Cursor& cursor = plot.cursor(i);
@@ -1730,34 +1737,143 @@ void Renderer::draw_cursors(const Plot& plot, const PlotLayout& layout) {
       flush_lines(mapping(plot, cursor.axis_x, 0, x_drawn),
                   on ? style_.cursor : cursor.color, width, cursor.dash);
     }
-    draw_cursor_label(plot, layout, cursor, drawn);
+    // The labels are drawn after the strokes, clustered: collecting them all first is what
+    // lets two marks in the same column share one panel instead of overprinting each other.
+    collect_cursor_label(plot, layout, cursor, drawn, labels);
   }
+  draw_clustered_cursor_labels(layout, labels);
 }
 
-void Renderer::draw_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
-                                 f32 at) {
-  const f32 gap = style_.device(3.0f);
+void Renderer::collect_cursor_label(const Plot& plot, const PlotLayout& layout,
+                                    const Cursor& cursor, f32 at,
+                                    std::vector<CursorLabel>& labels) {
+  (void)layout;
   if (cursor.is_horizontal()) {
     // A ruler's text is the value it is *for*, written the way the y axis writes its own
     // tick labels, so a value read off a ruler reads exactly like the tick beside it. The
-    // caller's caption, when there is one, is staged first on the same line, so neither the
-    // caption nor the value has to be dropped -- and both stay one text run's worth of
-    // glyphs, since the pen advances per glyph either way.
-    const Vec2 pen{layout.axes.x + gap, at + style_.device(2.0f)};
+    // caller's caption, when there is one, leads the same line.
     char value[32];
     plot.y_axis(cursor.axis_y).format_value(cursor.y, value, sizeof value);
-    if (cursor.label.empty()) {
-      stage_text(glyphs_under_, pen, value, style_.axis_text);
-      return;
-    }
-    stage_text(glyphs_under_, pen, cursor.label.c_str(), style_.axis_text);
-    stage_text(glyphs_under_, {pen.x + font_backend_.measure(cursor.label.c_str()) + gap, pen.y},
-               value, style_.axis_text);
+    const std::string text =
+        cursor.label.empty() ? std::string(value) : cursor.label + " " + value;
+    CursorLabel label;
+    label.text = text;
+    label.border = cursor.color;
+    label.anchor = at;  // the ruler's own row: the badge is centred on it
+    label.w = font_backend_.measure(text.c_str());
+    label.horizontal = true;
+    labels.push_back(std::move(label));
     return;
   }
-  if (!cursor.label.empty()) {
-    stage_text(glyphs_under_, {at + gap, layout.axes.y + style_.device(2.0f)},
-               cursor.label.c_str(), style_.axis_text);
+  if (cursor.label.empty()) return;
+  CursorLabel label;
+  label.text = cursor.label;
+  label.border = cursor.color;
+  label.anchor = at;  // the marker's own column: the badge is centred on it
+  label.w = font_backend_.measure(cursor.label.c_str());
+  labels.push_back(std::move(label));
+}
+
+void Renderer::draw_clustered_cursor_labels(const PlotLayout& layout,
+                                            const std::vector<CursorLabel>& labels) {
+  if (labels.empty()) return;
+  const f32 pad = style_.device(3.0f);
+  const f32 line_h = font_backend_.height();
+  const Rect& axes = layout.axes;
+
+  // **Placement first, clustering second.** Each badge is centred on the mark it belongs to
+  // -- a ruler's badge straddles the ruler's own row, a marker's badge straddles the
+  // marker's own column -- because a badge that drifts from its mark is a number without an
+  // anchor. When two badges meet, the later one drops one text line (downward from its
+  // centred position) until it is clear, so a column of close rulers reads as a stack of
+  // numbers in their own rows rather than as texts printed through each other; the rows keep
+  // each badge beside its own mark, which a sequential stack from the panel's top would not.
+  std::vector<CursorLabel> placed;
+  placed.reserve(labels.size());
+  std::vector<CursorLabel> by_anchor = labels;
+  std::stable_sort(by_anchor.begin(), by_anchor.end(),
+                   [](const CursorLabel& a, const CursorLabel& b) { return a.anchor < b.anchor; });
+  for (CursorLabel& label : by_anchor) {
+    f32 x = 0.0f;
+    f32 y = 0.0f;
+    if (label.horizontal) {
+      x = axes.x + pad;
+      y = label.anchor - 0.5f * line_h;  // centred on the ruler's own row
+    } else {
+      // Centred on the marker's column, held inside the axes: a badge half over an edge is
+      // half a badge.
+      x = std::clamp(label.anchor - 0.5f * label.w, axes.x + pad,
+                     std::max(axes.x + pad, axes.right() - pad - label.w));
+      y = axes.y + pad;
+    }
+    for (;;) {
+      const Rect box{x - pad, y - pad, label.w + 2.0f * pad, line_h + 2.0f * pad};
+      bool meets = false;
+      for (const CursorLabel& p : placed) {
+        meets = box.x < p.box.x + p.box.w && p.box.x < box.x + box.w &&
+                box.y < p.box.y + p.box.h && p.box.y < box.y + box.h;
+        if (meets) break;
+      }
+      label.pen = Vec2{x, y};
+      label.box = box;
+      if (!meets) break;
+      y += line_h;  // one row down: still beside the mark, no longer on its neighbour
+    }
+    placed.push_back(label);
+  }
+
+  // The panels: one per cluster of meeting backplates -- the boxes' union plus padding, the
+  // border the cluster's first label's own -- with every line staged at its *placed* pen, so
+  // the panel is a unified plate behind badges that each still sit on their own mark.
+  const f32 margin = style_.device(2.0f);
+  std::vector<Rect> bounds;
+  std::vector<std::vector<std::size_t>> groups;
+  std::vector<char> used(placed.size(), 0);
+  for (std::size_t i = 0; i < placed.size(); ++i) {
+    if (used[i] != 0) continue;
+    used[i] = 1;
+    bounds.push_back(placed[i].box);
+    groups.emplace_back(1, i);
+    Rect& u = bounds.back();
+    for (bool grew = true; grew;) {
+      grew = false;
+      for (std::size_t j = 0; j < placed.size(); ++j) {
+        if (used[j] != 0) continue;
+        const Rect inflated{u.x - margin, u.y - margin, u.w + 2.0f * margin,
+                            u.h + 2.0f * margin};
+        const Rect& b = placed[j].box;
+        const bool meet = inflated.x < b.x + b.w && b.x < inflated.x + inflated.w &&
+                          inflated.y < b.y + b.h && b.y < inflated.y + inflated.h;
+        if (!meet) continue;
+        used[j] = 1;
+        groups.back().push_back(j);
+        const f32 rx = std::min(u.x, b.x);
+        const f32 ry = std::min(u.y, b.y);
+        u = Rect{rx, ry, std::max(u.x + u.w, b.x + b.w) - rx,
+                 std::max(u.y + u.h, b.y + b.h) - ry};
+        grew = true;
+      }
+    }
+  }
+  for (std::size_t g = 0; g < groups.size(); ++g) {
+    Rect panel = bounds[g];
+    panel.x -= pad;
+    panel.y -= pad;
+    panel.w += 2.0f * pad;
+    panel.h += 2.0f * pad;
+    // Inside the axes, shifted rather than clipped: a panel that hangs over an edge is a
+    // second plot; a panel nudged along it is still a readout of what it labels. Opaque: a
+    // badge is read against ticks and traces, and the badge's line beneath it is covered --
+    // the box interrupting its own ruler is what makes the two read as one instrument.
+    panel.x = std::clamp(panel.x, axes.x,
+                         std::max(axes.x, axes.right() - panel.w));
+    panel.y = std::clamp(panel.y, axes.y,
+                         std::max(axes.y, axes.bottom() - panel.h));
+    draw_readout_panel(panel, placed[groups[g].front()].border, 1.0f);
+    for (std::size_t k = 0; k < groups[g].size(); ++k) {
+      const CursorLabel& label = placed[groups[g][k]];
+      stage_text(glyphs_under_, label.pen, label.text.c_str(), style_.axis_text);
+    }
   }
 }
 
@@ -2023,7 +2139,7 @@ void Renderer::draw_legend(const Plot& plot, const PlotLayout& layout) {
   // The panel first, so every entry is on top of it. A legend's border is the frame's axis colour;
   // the readout's is the crosshair's own (see draw_crosshair_readout), which is the only thing that
   // differs between the two panels -- so it is the one parameter the shared helper takes.
-  if (plot.legend().frame) draw_readout_panel(legend.panel, style_.axis_line);
+  if (plot.legend().frame) draw_readout_panel(legend.panel, style_.axis_line, kLegendPanelAlpha);
 
   for (const LegendRow& row : legend.rows) {
     // The truncation row names no trace: it has no swatch to colour and its text is
@@ -2053,7 +2169,7 @@ void Renderer::draw_x_note(const Plot& plot, const PlotLayout& layout) {
   // A sliver of axes gets no caption rather than one hanging over the frame: the panel is chrome
   // *inside* the axes, and a backplate crossing the axis line would read as a second plot.
   if (panel.w > axes.w || panel.h > axes.h) return;
-  draw_readout_panel(panel, style_.axis_line);
+  draw_readout_panel(panel, style_.axis_line, kLegendPanelAlpha);
   // Bottom-left, beside the labels it qualifies -- the reader adds the caption to a tick, so it
   // belongs at the end of the tick row and not in a corner of its own. `glyphs_under_` because
   // the panel is flushed with the frame's line geometry and the under-text batch goes out after
@@ -2122,12 +2238,13 @@ void Renderer::draw_selection_frame(const PlotLayout& layout) {
   flush_lines(pixel_mapping(), style_.selection_edge, style_.device(1.0f));
 }
 
-void Renderer::draw_readout_panel(const Rect& panel, Color border) {
+void Renderer::draw_readout_panel(const Rect& panel, Color border, float background_alpha) {
   if (!(panel.w > 0.0f) || !(panel.h > 0.0f)) return;
-  // The frame's own background at a partial alpha, which dims whatever it covers instead of erasing
-  // it -- the one backplate both readout panels use.
+  // The frame's own background scaled by the caller's opacity, which dims whatever it covers
+  // -- or, at full opacity, covers it outright: a badge read against ticks or a noisy trace
+  // is read *through* nothing.
   Color background = style_.background;
-  background.a *= kLegendPanelAlpha;
+  background.a *= background_alpha;
   fill_pixels(panel, background);
   // The border frames the pixels the fill covers, not the measured rect's own fractional edges:
   // those are wherever the content happened to measure to, and a hairline drawn on one is lit
@@ -2226,14 +2343,47 @@ void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout)
   }
 }
 
+void Renderer::draw_crosshair_axis_badges(const PlotLayout& layout) {
+  const CrosshairLayout& crosshair = layout.crosshair;
+  if (!crosshair.active) return;
+  const bool y_badge = crosshair.y_axis_badge.w > 0.0f && !crosshair.y_axis_text.empty();
+  const bool x_badge = crosshair.x_axis_badge.w > 0.0f && !crosshair.x_axis_text.empty();
+  if (!y_badge && !x_badge) return;
+  // The same backplate the readout panel has, its border in the crosshair's colour too --
+  // the badge is that panel's value column, moved to where the line meets the axis it
+  // names. Drawn here rather than in the readout pass because it sits in a *label strip*:
+  // the strips are drawn after the interior clip is cleared, so a badge that lived in the
+  // clipped pass would be trimmed to nothing.
+  //
+  // **The tick labels are flushed first, and that is the whole point of this panel.** They
+  // are staged into the over-text batch, which the frame flushes after *every* line batch --
+  // so a fill drawn straight away would sit under them and the label it covers would print
+  // through the badge (the "transparent" backplate). They are already placed at this point,
+  // so pushing them out costs one batch: the fill then covers them, and the badge's own text
+  // is staged after both fills and leaves with the rest of the over batch, over everything.
+  flush_text(glyphs_over_);
+  const f32 pad = style_.device(4.0f);
+  if (y_badge) draw_readout_panel(crosshair.y_axis_badge, style_.crosshair, 1.0f);
+  if (x_badge) draw_readout_panel(crosshair.x_axis_badge, style_.crosshair, 1.0f);
+  if (y_badge) {
+    stage_text(glyphs_over_,
+               {crosshair.y_axis_badge.x + pad, crosshair.y_axis_badge.y + pad},
+               crosshair.y_axis_text.c_str(), style_.axis_text);
+  }
+  if (x_badge) {
+    stage_text(glyphs_over_,
+               {crosshair.x_axis_badge.x + pad, crosshair.x_axis_badge.y + pad},
+               crosshair.x_axis_text.c_str(), style_.axis_text);
+  }
+}
+
 void Renderer::draw_crosshair_readout(const Plot& plot, const PlotLayout& layout) {
   const CrosshairLayout& crosshair = layout.crosshair;
   if (crosshair.empty()) return;
-
   // The readout: the same backplate a legend has (the shared `draw_readout_panel()`), with its
   // border in the crosshair's own colour -- which is what makes the panel read as part of the
   // instrument that placed it rather than as a second legend.
-  draw_readout_panel(crosshair.panel, style_.crosshair);
+  draw_readout_panel(crosshair.panel, style_.crosshair, kLegendPanelAlpha);
 
   for (const CrosshairRow& row : crosshair.rows) {
     if (!row.listed) continue;