figure: shared-y rows draw one y axis, on the first cell

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

tests/test_figure.cpp

@@ -1341,3 +1341,70 @@ PLOT_TEST(figure_row_y_sharing_off_keeps_cells_independent) {
   PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).max, 1.0, 1e-9);
   PLOT_CHECK(!figure.share_row_y());
 }
+
+PLOT_TEST(figure_row_y_only_the_first_cell_has_a_gutter) {
+  plot::Figure figure = split_figure({1, 3}, false);
+  for (plot::u32 i = 0; i < figure.plot_count(); ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_share_row_y(true);
+  figure.layout();
+  PLOT_CHECK(figure.plot(0u).layout().y_axis_width > 0.0f);   // unsplit row keeps its axis
+  PLOT_CHECK(figure.plot(1u).layout().y_axis_width > 0.0f);   // first cell of the split row
+  for (plot::u32 i = 2; i < 4u; ++i) {
+    const plot::PlotLayout& l = figure.plot(i).layout();
+    PLOT_CHECK(l.y_axis_width == 0.0f && l.y_title_width == 0.0f && l.y_label_area.w == 0.0f);
+    PLOT_CHECK(l.axes.w > 0.0f);
+  }
+  // The gutter is one figure-wide value: the unsplit row and the split row's first cell agree.
+  PLOT_CHECK_NEAR(figure.plot(0u).layout().axes.x - figure.layout().cells[0].x,
+                  figure.plot(1u).layout().axes.x - figure.layout().cells[1].x, 0.01);
+}
+
+PLOT_TEST(figure_row_y_equal_weights_give_equal_plot_widths_and_cells_tile) {
+  plot::Figure figure = split_figure({3}, false);
+  for (plot::u32 i = 0; i < 3u; ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_col_gap(8.0f);
+  figure.set_share_row_y(true);
+  const plot::FigureLayout& fl = figure.layout();
+  const double w0 = figure.plot(0u).layout().axes.w;
+  PLOT_CHECK_NEAR(figure.plot(1u).layout().axes.w, w0, 0.6);
+  PLOT_CHECK_NEAR(figure.plot(2u).layout().axes.w, w0, 0.6);
+  PLOT_CHECK_NEAR(fl.cells[0].x, fl.rows[0].x, 0.001);
+  PLOT_CHECK_NEAR(fl.cells[2].right(), fl.rows[0].right(), 0.001);
+  PLOT_CHECK_NEAR(fl.cells[1].x - fl.cells[0].right(), 8.0, 0.01);
+  PLOT_CHECK_NEAR(fl.cells[2].x - fl.cells[1].right(), 8.0, 0.01);
+}
+
+PLOT_TEST(figure_row_y_weights_apply_to_plot_widths) {
+  plot::Figure figure = split_figure({2}, false);
+  for (plot::u32 i = 0; i < 2u; ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_col_gap(0.0f);
+  figure.set_share_row_y(true);
+  const std::vector<plot::f32> w{3.0f, 1.0f};
+  PLOT_CHECK(figure.set_cell_weights(0u, w));
+  figure.layout();
+  const double a0 = figure.plot(0u).layout().axes.w;
+  const double a1 = figure.plot(1u).layout().axes.w;
+  PLOT_CHECK_NEAR(a0 - 64.0, (a1 - 64.0) * 3.0, 1.0);
+}
+
+PLOT_TEST(figure_row_y_narrow_row_degrades_without_negative_rects) {
+  plot::Figure figure = split_figure({4}, false);
+  figure.set_viewport(plot::Rect{0.0f, 0.0f, 30.0f, 100.0f});
+  for (plot::u32 i = 0; i < 4u; ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_col_gap(10.0f);
+  figure.set_share_row_y(true);
+  double sum = 0.0;
+  for (const plot::Rect& c : figure.layout().cells) {
+    PLOT_CHECK(c.w >= 0.0f && c.h >= 0.0f);
+    sum += c.w;
+  }
+  PLOT_CHECK_NEAR(sum, 30.0, 0.05);
+}
+
+PLOT_TEST(figure_row_y_sharing_off_still_gives_every_cell_a_gutter) {
+  plot::Figure figure = split_figure({2}, false);
+  give_magnitude_row(figure.plot(0u), 0u);
+  give_magnitude_row(figure.plot(1u), 1u);
+  figure.layout();
+  PLOT_CHECK(figure.plot(1u).layout().y_axis_width > 0.0f);
+}

tests/test_render_smoke.cpp

@@ -5059,6 +5059,52 @@ PLOT_TEST(figure_x_title_is_drawn_on_the_bottom_row_only) {
 
   ctx.destroy();
 }
+/// A split row that shares one y axis draws it once, on its first cell: every later cell is
+/// `YStrip::None`, reserves no y strip, and the renderer must stage no y tick labels for it.
+/// Staged text is not observable through `FrameStats`, so this reads the pixels. The column gap
+/// is wide and empty chrome, and a label staged for the second cell would be right-aligned
+/// against that cell's (zero-width) label area, i.e. land in the gap: the gap holds no ink.
+PLOT_TEST(figure_shared_y_second_cell_stages_no_y_labels) {
+  plottest::HeadlessContext ctx(kW, kH);
+  plot::Status st;
+  PLOT_CHECK(ctx.create(&st));
+  PLOT_CHECK(plot::gl::load(nullptr));
+  plot::Renderer r;
+  PLOT_CHECK(r.init(&st));
+
+  const std::vector<plot::u32> cells{2u};
+  plot::Figure figure{std::span<const plot::u32>(cells)};
+  figure.set_viewport({0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  for (plot::u32 i = 0; i < 2u; ++i) {
+    plot::Plot& p = figure.plot(i);
+    p.add_x_axis({"t", {0.0, 100.0}, false, plot::AxisPlace::Bottom, false});
+    p.add_y_axis({"", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+  }
+  figure.set_col_gap(80.0f);
+  figure.set_share_row_y(true);
+
+  r.begin_frame(kW, kH, {0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  r.draw(figure);
+  r.end_frame();
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+  const plot::FigureLayout layout = figure.layout();
+  const plot::Color ink = r.style().axis_text;
+
+  const plot::PlotLayout& first = figure.plot(0u).layout();
+  const plot::PlotLayout& second = figure.plot(1u).layout();
+  PLOT_CHECK(second.y_label_area.w == 0.0f);
+  PLOT_CHECK(layout.cells[1].x - layout.cells[0].right() > 70.0f);
+  // The first cell still draws the row's one axis ...
+  PLOT_CHECK(scan_rect_ink(ctx, first.y_label_area, ink).pixels > 0u);
+  // ... and nothing is staged in the gap on the second cell's left.
+  // Inset a few pixels each side: the cells' own frame lines sit on the gap's two edges.
+  const plot::Rect gap{layout.cells[0].right() + 4.0f, first.axes.y,
+                       layout.cells[1].x - layout.cells[0].right() - 8.0f, first.axes.h};
+  PLOT_CHECK(scan_rect_ink(ctx, gap, ink).pixels == 0u);
+
+  ctx.destroy();
+}
+
 // ---------------------------------------------------- the axes clip (spec §13)
 
 // The reported defect this section is about: a trace is staged in *data* space and