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

src/plot/figure.cpp

@@ -393,12 +393,41 @@ const FigureLayout& Figure::layout() {
   // its transforms describe this frame's rects; and before the rows lay themselves
   // out below, so a following group's window is the range they lay out with.
   sync_x_links();
-  // Every row is laid out against its own rect here, so after this call each row's
+  // Every row is laid out against its own rect, so after this call each row's
   // chrome is current: `plot(i).layout()` reads back what the figure computed rather
   // than what the row last held. That is the contract `Renderer::draw(Figure&)`
   // relies on -- it draws each row with the layout this produced, and `draw(Plot&)`
   // is the one that computes its own.
-  for (u32 row = 0; row < rows; ++row) plots_[row].layout();
+  //
+  // The rows' axes have to line up (spec §16), so this is where the figure's **common
+  // left gutter** is decided -- a row cannot see its siblings' tick labels, so the one
+  // width they can agree on has to be handed to them from here.
+  //
+  // Two passes, and no more. The first lays every row out with no floor, so each
+  // reports the gutter its own labels and title column need; the figure keeps the
+  // maximum. The second re-lays every row with that maximum imposed on its
+  // `Plot::set_min_y_gutter()`, which widens a row's strip to fill it, so all the rows'
+  // `axes` start and end on the same columns. A fixed-point loop -- re-measuring until
+  // the rows stop moving -- is the tempting wrong answer and is not needed: the gutter
+  // is a *maximum* over rows, and a row's own strip width does not depend on any other
+  // row's or on the width imposed on it (y tick labels are chosen from the row's own
+  // range and height, and a title column is one text line per titled axis), so one read
+  // per row settles the answer exactly. Measuring with a floor from the previous frame
+  // would be the only reason to iterate, and clearing it before the measurement pass is
+  // what removes it.
+  f32 gutter = 0.0f;
+  for (u32 row = 0; row < rows; ++row) {
+    plots_[row].set_min_y_gutter(0.0f);
+    const PlotLayout& row_layout = plots_[row].layout();
+    gutter = std::max(gutter, row_layout.y_axis_width + row_layout.y_title_width);
+  }
+  // A one-row figure imposes exactly what that row measured, so it lays out as the plot
+  // would on its own; a figure whose rows all need the same strip imposes that, which is
+  // also what each would have done alone.
+  for (u32 row = 0; row < rows; ++row) {
+    plots_[row].set_min_y_gutter(gutter);
+    plots_[row].layout();
+  }
 
   // The figure's one x axis is the bottom row's, so the strip it is drawn in is that
   // row's own label area -- the rect the controller hit-tests and the renderer stages