fix(plot): a figure gives every row the same y gutter so the axes line up

include/plot/plot/figure.hpp

@@ -136,6 +136,14 @@ class Figure {
   /// This is also where a row's `PlotLayout` becomes well-defined: a row's chrome is
   /// laid out against `rows[i]` by its own `Plot::layout()`, which is why the figure
   /// computes rects and not axes.
+  ///
+  /// It is also the one place the rows' left chrome can be made to agree (spec §16):
+  /// each row is laid out once to learn the gutter its own labels and title column
+  /// need, the figure takes the maximum and imposes it on every row through
+  /// `Plot::set_min_y_gutter()` before laying the rows out once more -- so every row's
+  /// `axes.x` and `axes.w` are equal and a vertical feature lands in the same pixel
+  /// column in each. Two passes per row per call, not a loop: the gutter is a maximum
+  /// over rows and a row's own strip width does not depend on it.
   const FigureLayout& layout();
   const FigureLayout& layout() const noexcept { return layout_; }
 

include/plot/plot/plot.hpp

@@ -524,6 +524,24 @@ class Plot {
   /// a lookup on a const Plot cannot move the axes out from under the caller.
   const PlotLayout& layout() const noexcept { return layout_; }
 
+  /// 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.
+  ///
+  /// 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
+  /// learn the gutter it needs, takes the maximum and sets it here on every row --
+  /// after which all the rows' `axes.x` and `axes.w` agree and a vertical feature
+  /// lands in the same pixel column in each. The floor is applied inside the same cap
+  /// as the measured strip, so a figure's gutter can never squeeze `axes` below the
+  /// half-viewport floor either. Negative values are clamped to 0.
+  void set_min_y_gutter(f32 device_px) noexcept {
+    min_y_gutter_ = device_px > 0.0f ? device_px : 0.0f;
+  }
+  f32 min_y_gutter() const noexcept { return min_y_gutter_; }
+
   /// Grid segments for the primary axis pair (x axis 0, y axis 0), in data space.
   const Grid& grid() const noexcept { return grid_; }
 
@@ -713,6 +731,9 @@ class Plot {
   Legend legend_{};
   /// Which parts of the bottom x chrome are reserved at all; see set_x_strip_mode().
   XStrip x_strip_ = XStrip::Both;
+  /// The floor a figure imposes on the left gutter; see set_min_y_gutter(). 0.0 means
+  /// none, which is what a plot standing on its own keeps.
+  f32 min_y_gutter_ = 0.0f;
   /// Which handles a pointer is on, published by the input layer (see HotHandles).
   HotHandles hot_{};