feat(plot): Figure with stacked rows and a synchronised x axis

src/plot/plot.cpp

@@ -661,6 +661,12 @@ const PlotLayout& Plot::layout() {
     y_width = y_axes_.empty() ? 0.0f : 2.0f * pad;
     x_height = text_->height() + 2.0f * pad;
   }
+  // A row of a `Figure` gives its x strip to the figure's one shared axis, so it
+  // reserves none: the seed and the fixed point below both start from zero, and
+  // `axes` then reaches the viewport's bottom edge. Set here rather than in the
+  // loop, because the loop's `need_x` is what the strip wants and this overrides
+  // the want, not the measurement.
+  if (!x_label_strip_) x_height = 0.0f;
   // The title strips, though, are needed before the loop and never change, unlike
   // the label strips: a strip holds exactly one text line per titled axis and no
   // tick label can change that. Nothing at all is reserved without a text backend
@@ -670,7 +676,7 @@ const PlotLayout& Plot::layout() {
   // per titled y axis *wide*, because the y title is drawn rotated and so spends
   // its length along the axes' height and takes a column, not a row, per titled
   // axis (see y_title_strip_width).
-  const u32 x_titles = titled_x_axis_count();
+  const u32 x_titles = x_label_strip_ ? titled_x_axis_count() : 0u;
   const f32 y_title_w = text_ != nullptr ? y_title_strip_width() : 0.0f;
   const f32 x_title_h = text_ != nullptr && x_titles != 0u
                             ? static_cast<f32>(x_titles) * text_->height() + 2.0f * pad
@@ -710,8 +716,12 @@ const PlotLayout& Plot::layout() {
     // The same caps as above, so this compares like with like and the loop still
     // reaches its fixed point on the second pass.
     const f32 need_y = std::min(std::clamp(y_label_width(), 0.0f, viewport_.w), y_budget);
+    // Overridden, not merely unmeasured: the strip is not reserved, so the fixed
+    // point has to be zero for a row that gave its strip away.
     const f32 need_x =
-        std::min(std::clamp(text_->height() + 2.0f * pad, 0.0f, viewport_.h), x_budget);
+        x_label_strip_
+            ? std::min(std::clamp(text_->height() + 2.0f * pad, 0.0f, viewport_.h), x_budget)
+            : 0.0f;
     if (need_y == y_width && need_x == x_height) break;  // strips are a fixed point
     y_width = need_y;
     x_height = need_x;