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_{};

include/plot/render/renderer.hpp

@@ -261,15 +261,22 @@ class Renderer {
   /// Lays one run of text out at a framebuffer position and stages it, shifting
   /// it into viewport space. Returns the pen past the run.
   Vec2 stage_text(Vec2 pos_px, const char* utf8);
-  /// Width of the longest prefix of `utf8` that fits `budget_px`, at the size the
-  /// text pipeline draws with: exactly what `stage_text_clipped()` stages.
-  f32 clipped_text_width(const char* utf8, f32 budget_px) const;
-  /// Stages as much of `utf8` as fits `budget_px` at `pos_px` and returns the
-  /// width it staged. Titles are the runs that need this: a title longer than the
-  /// strip the layout reserved for it is cut there, rather than being allowed to
-  /// grow the strip (which would shrink the plot) or to spill over the tick labels
-  /// and the data (which is the defect the strips exist to prevent).
-  f32 stage_text_clipped(Vec2 pos_px, const char* utf8, f32 budget_px);
+  /// Width of the longest prefix of `utf8` that fits `budget_px` along `axis`, at
+  /// the size the text pipeline draws with: exactly what `stage_text_clipped()`
+  /// stages.
+  f32 clipped_text_width(const char* utf8, f32 budget_px, Vec2 axis) const;
+  /// Stages as much of `utf8` as fits `budget_px` along `axis` at `pos_px` and
+  /// returns the length it staged. Titles are the runs that need this: a title
+  /// longer than the strip the layout reserved for it is cut there, rather than
+  /// being allowed to grow the strip (which would shrink the plot) or to spill
+  /// over the tick labels and the data (which is the defect the strips exist to
+  /// prevent).
+  ///
+  /// `axis` is the run's own direction and a unit vector: (1, 0) for the
+  /// horizontal x title, which spends the strip's width, and (0,-1) for the y
+  /// title, which reads bottom-to-top and spends the strip's height. `pos_px` is
+  /// the pen -- for the rotated run, the bottom-left corner of its first cell.
+  f32 stage_text_clipped(Vec2 pos_px, const char* utf8, f32 budget_px, Vec2 axis);
 
   void draw_grid(const Plot& plot);
   void draw_axis_lines(const Plot& plot);