feat(plot): vertical y-axis label via a rotated text run

include/plot/plot/plot.hpp

@@ -27,7 +27,12 @@ struct PlotLayout {
   Rect axes;
   Rect y_label_area;
   Rect x_label_area;
-  /// One text line each, just outside the matching label area. Zero-sized (and
+  /// The y title's strip is a *column*: one text line wide, the axes' height
+  /// tall, just outside `y_label_area`. The y title is drawn rotated to read
+  /// bottom-to-top, so what it needs is a line's height rather than the run's own
+  /// width -- a horizontal run's width is what this strip had to reserve before,
+  /// and a long title then ate the plot. The x title stays horizontal and gets a
+  /// row of the same height below `x_label_area`. Both are zero-sized (and
   /// unoccupied) when no axis of that direction has a title.
   Rect y_title_area;
   Rect x_title_area;
@@ -39,10 +44,10 @@ struct PlotLayout {
   /// is reserved. Every titled axis of a direction gets its own text line inside
   /// its strip -- which is what keeps a Left/Right (or Bottom/Top) pair from
   /// drawing two titles over each other -- so `x_title_height` is one line per
-  /// titled x axis; `y_title_width` is the widest y title plus the padding, since
-  /// y titles are horizontal runs that stack down the axes-tall strip and need
-  /// width, not another row of it. With no text backend there is no title to
-  /// measure and both stay 0.0f.
+  /// titled x axis and `y_title_width` is one line per titled y axis too: a
+  /// rotated title occupies length along the axes' height, so a second titled y
+  /// axis takes a *column* beside the first rather than another row of width. With
+  /// no text backend there is no title to lay out at all and both stay 0.0f.
   f32 y_title_width = 0.0f;
   f32 x_title_height = 0.0f;
 };
@@ -254,8 +259,9 @@ class Plot {
   /// Widest y tick label plus the label padding on both sides: the left strip the
   /// text backend asks for. Zero with no backend or no y axis.
   f32 y_label_width() const;
-  /// Widest y axis title plus the label padding on both sides: the vertical strip
-  /// a horizontal y title fits in. Zero with no backend or no titled y axis.
+  /// One text line of width per titled y axis, plus the label padding on both
+  /// sides: the column the rotated y title is drawn in, one column per titled
+  /// axis. Zero with no backend or no titled y axis.
   f32 y_title_strip_width() const;
 
   Rect viewport_{};