docs: correct three header comments that had drifted from the code

include/plot/plot/crosshair.hpp

@@ -22,7 +22,9 @@ namespace plot {
 /// and `dash_off` are independent, so a caller may override only the dash, or only the
 /// width, and leave the rest to the style.
 struct CrosshairStyle {
-  f32 width = -1.0f;     ///< logical stroke width; `< 0` takes `Style::crosshair_width`.
+  f32 width = -1.0f;     ///< logical stroke width; `< 0` takes `Style::crosshair_width`
+                         ///< (the default). Only a *negative* value defers: `0` is used
+                         ///< literally and draws an invisible line.
   f32 dash_on = -1.0f;   ///< logical ink length; `< 0` takes `Style::crosshair_dash_on`.
   f32 dash_off = -1.0f;  ///< logical gap; `< 0` takes `Style::crosshair_dash_off`, and `0`
                          ///< (once resolved) means solid.

include/plot/plot/plot.hpp

@@ -554,9 +554,11 @@ class Plot {
 
   /// A floor on the left gutter `layout()` reserves, in device pixels: the label
   /// strip and the title column together are never narrower than this, so `axes`
-  /// never starts left of `viewport().x + min_y_gutter()`. 0.0 -- the default --
-  /// imposes nothing and the plot sizes its strip from its own widest tick label,
-  /// which is what a plot standing on its own has always done.
+  /// starts no further left than `viewport().x + min_y_gutter()` **up to the cap
+  /// below** -- a floor wider than half the viewport is clamped exactly as the
+  /// measured strip is, so the guarantee is `min(min_y_gutter(), y_budget)`. 0.0
+  /// -- the default -- imposes nothing and the plot sizes its strip from its own
+  /// widest tick label, which is what a plot standing on its own has always done.
   ///
   /// It exists for `Figure`, which is the only thing that can know the answer: a row
   /// cannot see its siblings' tick labels, so the figure lays each row out once to