feat(plot): Figure with stacked rows and a synchronised x axis

include/plot/plot/plot.hpp

@@ -42,7 +42,10 @@ struct PlotLayout {
   Rect x_title_area;
   /// Left strip width. The default is used when no text backend has been set.
   f32 y_axis_width = 56.0f;
-  /// Bottom strip height. The default is used when no text backend has been set.
+  /// Bottom strip height. The default is used when no text backend has been set, and
+  /// it is 0.0f when the strip is not reserved at all -- see
+  /// `Plot::set_x_label_strip()`, which is how a row of a `Figure` hands the band to
+  /// the figure's one shared axis.
   f32 x_axis_height = 22.0f;
   /// Width of `y_title_area` and height of `x_title_area`, both 0.0f when nothing
   /// is reserved. Every titled axis of a direction gets its own text line inside
@@ -131,6 +134,24 @@ class Plot {
   /// The scale the last layout was computed at. 1.0 until a host says otherwise.
   f32 ui_scale() const noexcept { return style_.ui_scale; }
 
+  /// Whether `layout()` reserves the bottom x label strip -- and, with it, the x
+  /// title strip: a title is the labels' caption and goes where they do.
+  ///
+  /// True by default, which is what a plot drawn on its own has always been. A
+  /// `Figure` turns it off on every row while it draws one shared x axis under its
+  /// last row (see `Figure::set_x_labels_only_on_bottom()`): the strip is not
+  /// merely left blank, it is not reserved at all, so such a row's `axes` reaches
+  /// the row rect's bottom edge and the row gives the band to the figure's shared
+  /// axis instead of growing a second, duplicate one. Nothing else about the row
+  /// changes -- its ticks still refresh and its grid still builds, because the
+  /// tick list is what the shared axis and the grid are drawn from, not the strip.
+  ///
+  /// The plot it belongs to draws no x tick labels while it is off (their strip
+  /// has no height left to hold them); whoever turned it off is the one that puts
+  /// them somewhere -- `Renderer::draw(Figure&)` draws them in the figure's strip.
+  void set_x_label_strip(bool on) noexcept { x_label_strip_ = on; }
+  bool x_label_strip() const noexcept { return x_label_strip_; }
+
   /// The font layout() measures tick labels with. Borrowed, not owned, and may be
   /// null -- then the label areas fall back to PlotLayout's defaults and no string
   /// is measured: a plot with no renderer lays out with whatever the host sets
@@ -534,6 +555,8 @@ class Plot {
   render::Style style_{};
   const render::TextBackend* text_ = nullptr;
   Legend legend_{};
+  /// Whether the bottom x label strip is reserved at all; see set_x_label_strip().
+  bool x_label_strip_ = true;
   /// Which handles a pointer is on, published by the input layer (see HotHandles).
   HotHandles hot_{};