figure: x axes follow the column structure; split rows keep the right gutter on the last cell only

include/plot/plot/figure.hpp

@@ -32,15 +32,16 @@ struct FigureLayout {
   /// rect here; a row of several tiles its rect left to right with `col_gap` between
   /// cells, the cells' widths summing to the row's less the gaps.
   std::vector<Rect> cells;
-  /// The strip the figure's one shared x axis is drawn in: the bottom row's own
+  /// The strip the figure's bottom x axis is drawn in: the bottom row's own
   /// `PlotLayout::x_label_area` (for a bottom row split into several cells, only the **last
   /// cell's** strip -- the others' strips are their own `plot(i).layout().x_label_area`, which
-  /// is where the controller's axis-zoom hit test reads each cell's gestures from), at the bottom of the bottom row's rect and so as
-  /// wide as that row's axes rather than the whole figure. Read back from the row
+  /// is where the controller's axis-zoom hit test reads each cell's gestures from), at the
+  /// bottom of the bottom row's rect and so as wide as that row's axes rather than the whole
+  /// figure. Read back from the row
   /// rather than derived here, so a host and the frame cannot disagree about where
   /// the axis is. It is nonzero whenever the figure has rows and the bottom row
-  /// reserves a label strip -- including the case where every row labels itself, in
-  /// which case it is simply the bottom row's own strip, one of several.
+  /// reserves a label strip. A row above whose columns differ from the row below keeps
+  /// a strip of its own (see `Figure`), which is its plots' `x_label_area`, not this.
   ///
   /// It is the rect the controller's axis-zoom hit test reads (`x_label_area` is what
   /// a middle drag in a label strip enters `DragMode::AxisZoomX` from), which is why
@@ -70,12 +71,15 @@ struct FigureLayout {
 /// **The shared x axis.** Row heights are an equal division of the figure by
 /// default, `set_row_weights()` gives a row more of it, and every row keeps a floor
 /// height so chrome can never erase a plot. When `set_x_labels_only_on_bottom()` is
-/// on -- the default -- the figure's one x axis is the **bottom row's own**: the rows
-/// above reserve no x chrome at all, and the bottom row keeps its own label and title
-/// strips, so those labels and that title are drawn once, from the bottom row's own
-/// ticks and transform. That places the shared axis *inside* a row, where the
-/// controller's axis-zoom hit test can reach it, instead of in a band outside every
-/// row that no gesture could land in.
+/// on -- the default -- the x axis is the **bottom row's own** for every run of rows
+/// with the *same columns* (the same number of cells and the same normalised cell
+/// weights): those rows above reserve no x chrome at all, and the bottom row of the run
+/// keeps its own label and title strips, so those labels and that title are drawn once,
+/// from its own ticks and transform. A row whose columns differ from the row below keeps
+/// its own strip, drawn under each of its cells, because the axis below is at another
+/// scale and width. The rule is structural, never a function of what a plot holds. That
+/// places a shared axis *inside* a row, where the controller's axis-zoom hit test can
+/// reach it, instead of in a band outside every row that no gesture could land in.
 ///
 /// **The x link.** `plot(i).set_x_range()` and a controller gesture both reach a
 /// member's x range the same way, so a figure cannot see either of them happen.
@@ -186,14 +190,17 @@ class Figure {
   /// than row by row for the left gutter's reason: the rows' axes have to line up, and
   /// they end on a common column only if they are told one width. See
   /// `Plot::set_right_gutter()` for what it is for. Negative values are clamped to 0.
+  /// In a split row whose cells share one y (`set_share_row_y`) only the **last** cell
+  /// keeps it, taken off the row like the y gutter, so the gap between two plots of the
+  /// row is the column gap.
   void set_right_gutter(f32 device_px) noexcept {
     right_gutter_ = device_px > 0.0f ? device_px : 0.0f;
   }
   f32 right_gutter() const noexcept { return right_gutter_; }
 
-  /// Whether the shared x axis is the figure's: every row reserves no x label strip
-  /// and one shared axis is drawn under the last row (the default), or every row
-  /// draws its own labels and the figure draws nothing extra.
+  /// Whether rows with the same columns as the row below hand it their x axis (the
+  /// default; a row with different columns keeps its own), or every row draws its own
+  /// labels and the figure draws nothing extra.
   void set_x_labels_only_on_bottom(bool on) noexcept { labels_bottom_only_ = on; }
   bool x_labels_only_on_bottom() const noexcept { return labels_bottom_only_; }
 

include/plot/render/renderer.hpp

@@ -238,12 +238,13 @@ class Renderer {
   /// exactly as a stand-alone plot is, and `draw(Plot&)` itself is unchanged for
   /// single-plot hosts.
 
-  /// The figure's one shared x axis is the **bottom row's own**, so it is drawn by
-  /// that row's normal path: when the figure says labels belong on the bottom only
-  /// (the default), the rows above reserve no x chrome and the bottom row stages its
-  /// own labels and title once, inside its own strip -- the rect
-  /// `FigureLayout::x_axis_strip` reads back. With that switched off, every row keeps
-  /// its own chrome and each draws its own labels; there is then no shared axis.
+  /// A shared x axis is the **bottom row's own** (of a run of rows with the same
+  /// columns), so it is drawn by that row's normal path: when the figure says labels
+  /// belong on the bottom only (the default), the rows above that match the row below
+  /// reserve no x chrome and the bottom row stages its own labels and title once,
+  /// inside its own strip -- the rect `FigureLayout::x_axis_strip` reads back. A row
+  /// whose columns differ keeps and draws its own; with that switched off, every row
+  /// does and there is no shared axis.
 
   /// Styling, the text backend and `ui_scale` are installed on **every** row before
   /// the figure computes its layout, so the strips the rows measure and the strokes

src/plot/figure.cpp

@@ -73,6 +73,38 @@ f32 place_cells(const Rect& r, u32 n, const std::vector<f32>& weights, f32 col_g
   return gap;
 }
 
+/// The share of the row each of its `n` cells is given by `weights`, exactly as `place_cells`
+/// reads them: an empty (or wrongly sized, or non-positive-total) vector means equal shares.
+std::vector<f32> normalised_shares(const std::vector<f32>& weights, u32 n) {
+  std::vector<f32> shares(n, 1.0f / static_cast<f32>(n));
+  f32 total = 0.0f;
+  if (weights.size() == n) {
+    for (const f32 w : weights) total += w;
+  }
+  if (total > 0.0f) {
+    for (u32 i = 0; i < n; ++i) shares[i] = weights[i] / total;
+  }
+  return shares;
+}
+
+/// Whether two rows have the **same columns**: the same number of cells and the same
+/// normalised cell weights (within 1e-4). The rows are all the figure's width, so such rows
+/// get cells with identical x extents -- the same y gutter, gap, floors and right gutter
+/// apply to both -- and one row's x axis serves the other. Structural, never content: it
+/// reads the cell counts and the weights the host set and nothing a plot holds.
+bool same_columns(const std::vector<u32>& cells_per_row,
+                  const std::vector<std::vector<f32>>& cell_weights, u32 row_a, u32 row_b) {
+  const u32 n = cells_per_row[row_a];
+  if (n != cells_per_row[row_b]) return false;
+  if (n == 1u) return true;
+  const std::vector<f32> a = normalised_shares(cell_weights[row_a], n);
+  const std::vector<f32> b = normalised_shares(cell_weights[row_b], n);
+  for (u32 i = 0; i < n; ++i) {
+    if (std::fabs(a[i] - b[i]) > 1e-4f) return false;
+  }
+  return true;
+}
+
 /// A coordinate in the shared axis' **axis space**: `log10` when the figure's one
 /// x axis -- the bottom row's, the row the shared strip belongs to -- is
 /// logarithmic, the value itself otherwise. A following group's span and offset
@@ -558,12 +590,13 @@ const FigureLayout& Figure::layout() {
   }
 
   const f32 figure_h = std::max(0.0f, viewport_.h);
-  // The rows tile the figure itself: the shared x axis is the *bottom row's own*, so
-  // nothing is taken off the figure for it. That is what keeps the axis reachable --
-  // its strip is a real `PlotLayout::x_label_area` inside the bottom row's rect, which
-  // is the rect the controller's axis-zoom hit test reads and the strip an x title is
-  // drawn in -- and it is why an interior row reserves no x chrome at all instead of
-  // leaving a band no one owns.
+  // The rows tile the figure itself: an x axis is a row's *own* strip (the bottom row's, and
+  // the strip of any row whose columns differ from the row below), so nothing is taken off
+  // the figure for it. That is what keeps the axis reachable -- its strip is a real
+  // `PlotLayout::x_label_area` inside a row's rect, which is the rect the controller's
+  // axis-zoom hit test reads and the strip an x title is drawn in -- and it is why a row
+  // that hands its axis to the one below reserves no x chrome at all instead of leaving a
+  // band no one owns.
   const f32 content_h = figure_h;
   // The gap is dropped, rather than squeezing the rows, when the figure cannot hold
   // it: a figure one pixel tall with four rows still gives every row a positive
@@ -633,18 +666,31 @@ const FigureLayout& Figure::layout() {
     if (n > 1u && used < col_gap_) layout_.col_gap = 0.0f;
   }
 
-  const u32 bottom_row = rows - 1u;
+  // Which rows carry their own x strip, decided structurally and before the first
+  // measurement pass so that pass and the final one see the same strips. The bottom row
+  // always has one. A row above hides its strip only when the row directly below has the
+  // same columns (see `same_columns()`), because then the cells of the two rows have the
+  // same x extents and the axis below serves this row too; the rule is transitive, since
+  // that row is itself either labelled or hidden in favour of one that matches it. A row
+  // whose columns differ from the one below -- `[1][2]` over `[3]` -- keeps its own strip,
+  // drawn under each of its cells, because the axis below is at another scale and width.
+  // A figure told to label every row keeps the strips everywhere.
+  std::vector<XStrip> strips(rows, XStrip::Both);
+  if (labels_bottom_only_) {
+    for (u32 row = rows - 1u; row-- > 0u;) {
+      // By induction the row below is labelled or itself hidden in favour of a matching
+      // row, so hiding this one on a match with it is the transitive rule.
+      if (same_columns(cells_per_row_, cell_weights_, row, row + 1u)) strips[row] = XStrip::None;
+    }
+  }
   for (u32 row = 0; row < rows; ++row) {
-    // One x axis for the whole figure, and it is the bottom row's: every plot keeps its
-    // x chrome -- labels, title and the strip the controller's axis zoom is found in --
-    // and every plot above the bottom row gives all of it to that row, so the labels and
-    // the title are not drawn once per row. A figure told to label every row keeps them
-    // everywhere, and then there is no shared axis to speak of: each plot has its own. A
-    // split bottom row labels each of its cells, each against its own x axis.
-    const XStrip strip = labels_bottom_only_ && row != bottom_row ? XStrip::None : XStrip::Both;
+    const XStrip strip = strips[row];
     // A split row whose cells share one y range also shares one y axis, drawn by its first
     // cell: every other cell reserves no y chrome at all (see `YStrip::None`).
     const bool shared_y = share_row_y_ && cells_per_row_[row] > 1u;
+    // ... and its right gutter is the last cell's alone, so the gap between two plots of the
+    // row is the column gap and not a gutter more (see the placement block below).
+    const u32 last = row_first_[row + 1u] - 1u;
     for (u32 i = row_first_[row]; i < row_first_[row + 1u]; ++i) {
       Plot& p = plots_[i];
       p.set_viewport(layout_.cells[i]);
@@ -653,7 +699,7 @@ const FigureLayout& Figure::layout() {
       // Before either pass below, and the same value for every plot: nothing is measured on
       // the right, so unlike the left gutter this needs no measurement pass -- the plots
       // end on a common column because they are all told the one width.
-      p.set_right_gutter(right_gutter_);
+      p.set_right_gutter(shared_y && i != last ? 0.0f : right_gutter_);
     }
   }
   // After the viewports, so a row's ticks and grid -- built from its axes rect -- and
@@ -708,18 +754,28 @@ const FigureLayout& Figure::layout() {
   // to hold the axis. The gutter is capped at half the row, so a row too narrow for it
   // degrades to ordinary cells instead of producing a negative rect. The cells still tile the
   // row: the first starts on its left edge and the last still ends on its right edge.
+  //
+  // The right gutter is taken off the same way and belongs to the **last** cell alone (the
+  // other cells were told none above): left to every cell, the visible gap between two
+  // plots would be the column gap plus a whole right gutter. Together with the y gutter it is
+  // capped at half the row, and never exceeds the figure's own right gutter.
   for (u32 row = 0; row < rows; ++row) {
     const u32 n = cells_per_row_[row];
     if (!share_row_y_ || n <= 1u) continue;
     const Rect& r = layout_.rows[row];
     const f32 g_eff = std::max(0.0f, std::min(gutter, r.w / 2.0f));
+    const f32 rg_eff = std::max(0.0f, std::min(right_gutter_, r.w / 2.0f - g_eff));
     Rect* cells = &layout_.cells[row_first_[row]];
-    const f32 used = place_cells(Rect{r.x + g_eff, r.y, r.w - g_eff, r.h}, n, cell_weights_[row],
-                                 col_gap_, floor_scale, cells);
+    const f32 used = place_cells(Rect{r.x + g_eff, r.y, r.w - g_eff - rg_eff, r.h}, n,
+                                 cell_weights_[row], col_gap_, floor_scale, cells);
     cells[0].x -= g_eff;
     cells[0].w += g_eff;
+    cells[n - 1u].w += rg_eff;
     layout_.col_gap = used < col_gap_ ? 0.0f : layout_.col_gap;
     for (u32 i = row_first_[row]; i < row_first_[row + 1u]; ++i) plots_[i].set_viewport(layout_.cells[i]);
+    // What the cell actually reserved, so a row too narrow for the whole gutter is not asked
+    // for more than it was given (the same value whenever the row has room).
+    plots_[row_first_[row + 1u] - 1u].set_right_gutter(rg_eff);
   }
   // A one-row figure imposes exactly what that row measured, so it lays out as the plot
   // would on its own; a figure whose rows all need the same strip imposes that, which is
@@ -729,12 +785,12 @@ const FigureLayout& Figure::layout() {
     p.layout();
   }
 
-  // The figure's one x axis is the bottom row's, so the strip it is drawn in is that
-  // row's own label area -- the rect the controller hit-tests and the renderer stages
-  // its labels in, read back here rather than derived again, so a host and the frame
-  // cannot disagree about where the shared axis is. It is zero-sized only when there
-  // are no rows at all: with `set_x_labels_only_on_bottom(false)` it is simply the last
-  // row's own strip, one of several.
+  // `x_axis_strip` is the bottom row's own label area (the last plot's) -- the rect the
+  // controller hit-tests and the renderer stages its labels in, read back here rather than
+  // derived again, so a host and the frame cannot disagree about where it is. Rows above
+  // that carry their own strip (their columns differ from the row below, or every row is
+  // labelled) have it in their plots' own `layout().x_label_area`. It is zero-sized only
+  // when there are no rows at all.
   layout_.x_axis_strip = plots_.back().layout().x_label_area;
   return layout_;
 }

src/plot/plot.cpp

@@ -1312,8 +1312,8 @@ const PlotLayout& Plot::layout() {
     y_width = y_axes_.empty() ? 0.0f : 2.0f * pad;
     x_height = text_->height() + 2.0f * pad;
   }
-  // A row of a `Figure` gives its x strip to the figure's one shared axis -- the
-  // bottom row's own strip, where that axis lives -- so it reserves none: the seed and
+  // A row of a `Figure` that hands its x axis to the row below (which has the same
+  // columns, so the same x extents) reserves no strip: the seed and
   // the fixed point below both start from zero, and `axes` then reaches the viewport's
   // bottom edge. Set here rather than in the loop, because the loop's `need_x` is what
   // the strip wants and this overrides the want, not the measurement.

src/render/renderer.cpp

@@ -480,10 +480,11 @@ void Renderer::draw(Figure& figure) {
   // than laying each row out a second time. Nothing draws before this: a row's
   // viewport is the figure's to set.
   //
-  // The figure's one x axis needs no pass of its own: it is the bottom row's own, so
-  // that row stages it in its own strip through `draw_tick_labels()` like any other
-  // plot's axis -- and every row above it reserved no x chrome, which is what keeps
-  // the labels from appearing once per row without a second staging here.
+  // A shared x axis needs no pass of its own: it is the bottom row's own (of a run of rows
+  // with the same columns), so that row stages it in its own strip through
+  // `draw_tick_labels()` like any other plot's axis -- and every row of the run above it
+  // reserved no x chrome, which is what keeps the labels from appearing once per row without
+  // a second staging here. A row whose columns differ stages its own.
   figure.layout();
   for (u32 row = 0; row < rows; ++row) {
     Plot& plot = figure.plot(row);
@@ -878,8 +879,8 @@ void Renderer::draw_tick_labels(const Plot& plot, const PlotLayout& layout) {
   const f32 line_h = font_backend_.height();
 
   // X labels: unless the plot gave its x label strip away -- a row of a `Figure`
-  // that reserves no x chrome, because the figure draws one shared axis in the bottom
-  // row's own strip instead -- in which case it has no strip to put them in and their
+  // that reserves no x chrome, because the row below has the same columns and draws the
+  // axis in its own strip instead -- in which case it has no strip to put them in and their
   // absence is the point. A plot with no strip would otherwise stage its labels just
   // above `axes`' bottom edge, i.e. over its own data. Centred under their tick on the
   // strip's single text line, which is the one rule for a plot's labels and a figure's

tests/test_figure.cpp

@@ -1257,10 +1257,11 @@ PLOT_TEST(figure_cells_share_x_link_and_crosshair) {
 }
 
 PLOT_TEST(figure_cells_x_labels_bottom_row_only) {
-  plot::Figure figure = split_figure({1, 2});
+  // Rows with the same columns share the bottom one's axis: {1,1,1} labels the last row only.
+  plot::Figure figure = split_figure({1, 1, 1});
   const plot::FigureLayout& fl = figure.layout();
   PLOT_CHECK(figure.plot(0u).layout().x_label_area.h == 0.0f);
-  PLOT_CHECK(figure.plot(1u).layout().x_label_area.h > 0.0f);
+  PLOT_CHECK(figure.plot(1u).layout().x_label_area.h == 0.0f);
   PLOT_CHECK(figure.plot(2u).layout().x_label_area.h > 0.0f);
   PLOT_CHECK_NEAR(fl.x_axis_strip.h, figure.plot(2u).layout().x_label_area.h, 0.001);
 }
@@ -1408,3 +1409,250 @@ PLOT_TEST(figure_row_y_sharing_off_still_gives_every_cell_a_gutter) {
   figure.layout();
   PLOT_CHECK(figure.plot(1u).layout().y_axis_width > 0.0f);
 }
+
+// ------------------------------------------------- x axes follow the columns
+
+namespace {
+
+/// A split figure with a text backend and a y axis of a real magnitude on every plot (so the
+/// shared y gutter is non-zero), the host's configuration: row-shared y, a 6 px column gap and
+/// a 35 px right gutter.
+plot::Figure host_like_figure(const std::vector<plot::u32>& cells, bool share_y = true) {
+  plot::Figure figure = split_figure(cells, false);
+  for (plot::u32 i = 0; i < figure.plot_count(); ++i) give_magnitude_row(figure.plot(i), i);
+  figure.set_col_gap(6.0f);
+  figure.set_right_gutter(35.0f);
+  figure.set_share_row_y(share_y);
+  return figure;
+}
+
+bool shows_x_axis(plot::Figure& figure, plot::u32 plot_index) {
+  const plot::PlotLayout& l = figure.plot(plot_index).layout();
+  return l.x_axis_height > 0.0f && l.x_label_area.h > 0.0f;
+}
+
+bool hides_x_axis(plot::Figure& figure, plot::u32 plot_index) {
+  const plot::PlotLayout& l = figure.plot(plot_index).layout();
+  return l.x_axis_height == 0.0f && l.x_label_area.h == 0.0f;
+}
+
+}  // namespace
+
+PLOT_TEST(figure_x_axis_a_split_row_over_an_unsplit_one_keeps_its_own) {
+  plot::Figure figure = host_like_figure({2, 1});
+  figure.layout();
+  PLOT_CHECK(shows_x_axis(figure, 0u));
+  PLOT_CHECK(shows_x_axis(figure, 1u));
+  PLOT_CHECK(shows_x_axis(figure, 2u));
+  // ... and the other way round.
+  plot::Figure other = host_like_figure({1, 2});
+  other.layout();
+  PLOT_CHECK(shows_x_axis(other, 0u));
+  PLOT_CHECK(shows_x_axis(other, 1u));
+  PLOT_CHECK(shows_x_axis(other, 2u));
+}
+
+PLOT_TEST(figure_x_axis_rows_with_the_same_columns_share_the_bottom_axis) {
+  plot::Figure figure = host_like_figure({2, 2});
+  figure.layout();
+  PLOT_CHECK(hides_x_axis(figure, 0u));
+  PLOT_CHECK(hides_x_axis(figure, 1u));
+  PLOT_CHECK(shows_x_axis(figure, 2u));
+  PLOT_CHECK(shows_x_axis(figure, 3u));
+  // The hidden row's cells have exactly the x extents of the cells below, so the axis below
+  // serves them.
+  const plot::FigureLayout& fl = figure.layout();
+  for (plot::u32 c = 0; c < 2u; ++c) {
+    PLOT_CHECK_NEAR(fl.cells[c].x, fl.cells[2u + c].x, 1e-4);
+    PLOT_CHECK_NEAR(fl.cells[c].w, fl.cells[2u + c].w, 1e-4);
+    const plot::PlotLayout& up = figure.plot(c).layout();
+    const plot::PlotLayout& down = figure.plot(2u + c).layout();
+    PLOT_CHECK_NEAR(up.axes.x, down.axes.x, 1e-4);
+    PLOT_CHECK_NEAR(up.axes.w, down.axes.w, 1e-4);
+    PLOT_CHECK_NEAR(up.y_axis_width, down.y_axis_width, 1e-4);
+  }
+  // The same without row-shared y (every cell has its own y strip).
+  plot::Figure own_y = host_like_figure({2, 2}, false);
+  own_y.layout();
+  PLOT_CHECK(hides_x_axis(own_y, 0u) && hides_x_axis(own_y, 1u));
+  PLOT_CHECK(shows_x_axis(own_y, 2u) && shows_x_axis(own_y, 3u));
+  for (plot::u32 c = 0; c < 2u; ++c) {
+    PLOT_CHECK_NEAR(own_y.plot(c).layout().axes.x, own_y.plot(2u + c).layout().axes.x, 1e-4);
+    PLOT_CHECK_NEAR(own_y.plot(c).layout().axes.w, own_y.plot(2u + c).layout().axes.w, 1e-4);
+  }
+}
+
+PLOT_TEST(figure_x_axis_matching_compares_normalised_weights) {
+  const std::vector<plot::f32> one_three{1.0f, 3.0f};
+  const std::vector<plot::f32> two_six{2.0f, 6.0f};
+  const std::vector<plot::f32> halves{1.0f, 1.0f};
+  {  // 1:3 over 1:3
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    PLOT_CHECK(figure.set_cell_weights(1u, one_three));
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+    PLOT_CHECK(shows_x_axis(figure, 2u) && shows_x_axis(figure, 3u));
+    const plot::FigureLayout& fl = figure.layout();
+    PLOT_CHECK_NEAR(fl.cells[0].w, fl.cells[2].w, 1e-3);
+    PLOT_CHECK_NEAR(figure.plot(1u).layout().axes.x, figure.plot(3u).layout().axes.x, 1e-3);
+  }
+  {  // 1:3 over 2:6 -- normalised, so the same columns
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    PLOT_CHECK(figure.set_cell_weights(1u, two_six));
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+  }
+  {  // 1:3 over 1:1
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    PLOT_CHECK(figure.set_cell_weights(1u, halves));
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u) && shows_x_axis(figure, 1u));
+    PLOT_CHECK(shows_x_axis(figure, 2u) && shows_x_axis(figure, 3u));
+  }
+  {  // 1:3 over (no weights = equal)
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u) && shows_x_axis(figure, 1u));
+  }
+  {  // explicit 1:1 over (no weights = equal)
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, halves));
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+  }
+  {  // a different number of cells never matches, whatever the weights
+    plot::Figure figure = host_like_figure({3, 2});
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u) && shows_x_axis(figure, 2u));
+  }
+}
+
+PLOT_TEST(figure_x_axis_unsplit_rows_are_unchanged) {
+  plot::Figure figure = host_like_figure({1, 1, 1});
+  figure.layout();
+  PLOT_CHECK(hides_x_axis(figure, 0u));
+  PLOT_CHECK(hides_x_axis(figure, 1u));
+  PLOT_CHECK(shows_x_axis(figure, 2u));
+  PLOT_CHECK_NEAR(figure.layout().x_axis_strip.h, figure.plot(2u).layout().x_label_area.h, 1e-4);
+}
+
+PLOT_TEST(figure_x_axis_follows_a_chain_of_matching_rows) {
+  {  // [2]/[2]/[1]: row 0 matches row 1 and hides; row 1 does not match row 2 and shows.
+    plot::Figure figure = host_like_figure({2, 2, 1});
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+    PLOT_CHECK(shows_x_axis(figure, 2u) && shows_x_axis(figure, 3u));
+    PLOT_CHECK(shows_x_axis(figure, 4u));
+  }
+  {  // [2]/[2]/[2]: transitive -- only the last row carries the axis.
+    plot::Figure figure = host_like_figure({2, 2, 2});
+    figure.layout();
+    for (plot::u32 i = 0; i < 4u; ++i) PLOT_CHECK(hides_x_axis(figure, i));
+    PLOT_CHECK(shows_x_axis(figure, 4u) && shows_x_axis(figure, 5u));
+  }
+  {  // [1]/[2]/[2]: the unsplit row matches nothing below it.
+    plot::Figure figure = host_like_figure({1, 2, 2});
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u));
+    PLOT_CHECK(hides_x_axis(figure, 1u) && hides_x_axis(figure, 2u));
+    PLOT_CHECK(shows_x_axis(figure, 3u) && shows_x_axis(figure, 4u));
+  }
+  {  // the figure's x_axis_strip is still the last plot's label area
+    plot::Figure figure = host_like_figure({2, 1});
+    const plot::FigureLayout& fl = figure.layout();
+    PLOT_CHECK(fl.x_axis_strip.h > 0.0f);
+    PLOT_CHECK_NEAR(fl.x_axis_strip.h, figure.plot(2u).layout().x_label_area.h, 1e-4);
+  }
+}
+
+PLOT_TEST(figure_x_axis_labels_on_every_row_when_asked) {
+  plot::Figure figure = host_like_figure({2, 2, 1});
+  figure.set_x_labels_only_on_bottom(false);
+  figure.layout();
+  for (plot::u32 i = 0; i < figure.plot_count(); ++i) PLOT_CHECK(shows_x_axis(figure, i));
+}
+
+// -------------------------------------------- the gap between cells of a row
+
+PLOT_TEST(figure_split_row_gap_between_plot_frames_is_the_column_gap) {
+  for (const plot::u32 n : {2u, 3u, 4u}) {
+    plot::Figure figure = host_like_figure({n});
+    const plot::FigureLayout& fl = figure.layout();
+    PLOT_CHECK_NEAR(fl.col_gap, 6.0, 1e-4);
+    for (plot::u32 k = 0; k + 1u < n; ++k) {
+      PLOT_CHECK_NEAR(figure.plot(k + 1u).layout().axes.x - figure.plot(k).layout().axes.right(),
+                      6.0, 1e-3);
+      PLOT_CHECK_NEAR(fl.cells[k + 1u].x - fl.cells[k].right(), 6.0, 1e-3);
+      // Only the last cell holds the right gutter.
+      PLOT_CHECK_NEAR(figure.plot(k).layout().right_gutter, 0.0, 1e-6);
+    }
+    // Equal weights give equal axes widths, the first cell included.
+    for (plot::u32 k = 1; k < n; ++k) {
+      PLOT_CHECK_NEAR(figure.plot(k).layout().axes.w, figure.plot(0u).layout().axes.w, 0.6);
+    }
+    // The cells tile the row exactly.
+    PLOT_CHECK_NEAR(fl.cells[0].x, fl.rows[0].x, 1e-4);
+    PLOT_CHECK_NEAR(fl.cells[n - 1u].right(), fl.rows[0].right(), 1e-3);
+    // The right gutter is still there, on the last cell.
+    const plot::PlotLayout& last = figure.plot(n - 1u).layout();
+    PLOT_CHECK_NEAR(fl.cells[n - 1u].right() - last.axes.right(), 35.0, 1e-3);
+    PLOT_CHECK_NEAR(last.right_gutter, 35.0, 1e-3);
+    // The y gutter still sits on the first cell alone.
+    PLOT_CHECK(figure.plot(0u).layout().axes.x > fl.cells[0].x);
+    PLOT_CHECK_NEAR(figure.plot(1u).layout().axes.x, fl.cells[1].x, 1e-4);
+  }
+}
+
+PLOT_TEST(figure_split_row_gap_with_weights_and_a_y_gutter) {
+  plot::Figure figure = host_like_figure({1, 2});
+  const std::vector<plot::f32> w{3.0f, 1.0f};
+  PLOT_CHECK(figure.set_cell_weights(1u, w));
+  const plot::FigureLayout& fl = figure.layout();
+  const plot::Rect a0 = figure.plot(1u).layout().axes;
+  const plot::Rect a1 = figure.plot(2u).layout().axes;
+  PLOT_CHECK_NEAR(a1.x - a0.right(), 6.0, 1e-3);
+  PLOT_CHECK_NEAR(a0.w - 64.0, (a1.w - 64.0) * 3.0, 1.0);
+  PLOT_CHECK_NEAR(fl.cells[2].right(), fl.rows[1].right(), 1e-3);
+  PLOT_CHECK_NEAR(fl.cells[1].x, fl.rows[1].x, 1e-4);
+  PLOT_CHECK_NEAR(fl.cells[2].right() - a1.right(), 35.0, 1e-3);
+  // The unsplit row above keeps its own right gutter on its one plot.
+  PLOT_CHECK_NEAR(fl.cells[0].right() - figure.plot(0u).layout().axes.right(), 35.0, 1e-3);
+}
+
+PLOT_TEST(figure_split_row_gutters_degrade_on_a_narrow_row) {
+  plot::Figure figure = host_like_figure({3});
+  figure.set_viewport(plot::Rect{0.0f, 0.0f, 60.0f, 100.0f});
+  const plot::FigureLayout& fl = figure.layout();
+  double sum = 0.0;
+  for (const plot::Rect& c : fl.cells) {
+    PLOT_CHECK(c.w >= 0.0f && c.h >= 0.0f);
+    sum += c.w;
+  }
+  PLOT_CHECK_NEAR(sum + fl.col_gap * 2.0, 60.0, 0.05);
+  PLOT_CHECK_NEAR(fl.cells[2].right(), 60.0, 0.05);
+  for (plot::u32 i = 0; i < 3u; ++i) PLOT_CHECK(figure.plot(i).layout().axes.w >= 0.0f);
+}
+
+PLOT_TEST(figure_row_gutter_without_sharing_is_the_old_layout) {
+  plot::Figure figure = host_like_figure({2}, false);
+  const plot::FigureLayout& fl = figure.layout();
+  // Plain cells: equal halves of the row less the gap, every plot with its own right gutter.
+  PLOT_CHECK_NEAR(fl.cells[0].w, (fl.rows[0].w - 6.0) / 2.0, 1e-3);
+  PLOT_CHECK_NEAR(fl.cells[1].w, (fl.rows[0].w - 6.0) / 2.0, 1e-3);
+  PLOT_CHECK_NEAR(fl.cells[1].x - fl.cells[0].right(), 6.0, 1e-4);
+  for (plot::u32 i = 0; i < 2u; ++i) {
+    PLOT_CHECK_NEAR(figure.plot(i).layout().right_gutter, 35.0, 1e-3);
+    PLOT_CHECK_NEAR(fl.cells[i].right() - figure.plot(i).layout().axes.right(), 35.0, 1e-3);
+  }
+  // An unsplit row is unchanged by sharing being on.
+  plot::Figure unsplit = host_like_figure({1, 1});
+  const plot::FigureLayout& ul = unsplit.layout();
+  for (plot::u32 i = 0; i < 2u; ++i) {
+    PLOT_CHECK_NEAR(ul.cells[i].w, ul.rows[i].w, 1e-4);
+    PLOT_CHECK_NEAR(ul.cells[i].right() - unsplit.plot(i).layout().axes.right(), 35.0, 1e-3);
+  }
+}