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