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

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);