fix(render): floor the axes so chrome cannot erase it, one title row per titled axis

Two findings on the pass-1 fix:

- The reserved strips were clamped only to the viewport, so a title wider than a
  narrow panel took the rest of it and the axes collapsed: draw_traces() returned
  on a zero-width rect (no trace drawn, nothing reported) and a zero-height one
  flattened every trace onto a single row. The chrome is now capped at half the
  viewport -- the label strip is served first, the title strip takes what is left
  of that allowance -- an over-long title is clipped to its strip by a walk that
  both measures and stages it, and draw_traces() tests the height as well as the
  width.
- Every titled y axis was staged at the same centred position inside one strip, so
  a Left/Right pair of titles overprinted. Each titled axis now owns a text line:
  the x strip grows one line per titled x axis (x_title_height), and the y runs
  stack down the axes-tall strip, which needs no extra width for it.

Tests: `narrow_panel_with_a_long_y_title_still_draws` and
`short_panel_with_an_x_title_still_draws` pin the floor (the axes keeps half the
panel and the trace still covers the columns, with the title clipped into its
strip), `titled_axis_pairs_get_their_own_rows` pins one text row per titled axis
in both directions, and `layout_stacks_one_title_line_per_titled_axis` pins the
strip sizing. All fail against the pass-1 sources.

src/plot/plot.cpp

@@ -11,6 +11,16 @@ namespace {
 /// changing, so a normal plot converges on the second pass and never reaches this.
 constexpr u32 kMaxLayoutPasses = 4;
 
+/// Share of the viewport the axes rect keeps however much chrome the label and
+/// title strips ask for. A long SI label or a title wider than a narrow panel can
+/// ask for the whole viewport, and an axes rect with no width or no height draws
+/// nothing at all -- no grid, no tick, no trace -- without reporting anything, so
+/// the strips give way instead. The label strip is what makes the tick values
+/// readable, so it is served first and the title strip takes what is left; a title
+/// longer than its share of the strip is clipped to it rather than allowed to grow
+/// it (see `Renderer::draw_titles()`).
+constexpr f32 kMinAxesShare = 0.5f;
+
 const Axis& default_axis() noexcept {
   static const Axis axis{};
   return axis;
@@ -311,22 +321,33 @@ const PlotLayout& Plot::layout() {
     x_height = text_->height() + 2.0f * pad;
   }
   // The title strips, though, are needed before the loop and never change, unlike
-  // the label strips: a strip holds exactly one text line, so no tick label can
-  // make it wider. Nothing at all is reserved without a text backend -- there is
-  // then no title to measure, which is what keeps the two defaults above
-  // meaningful for the no-text-backend case. The x strip is one text line tall;
-  // the y strip is one horizontal run wide (see y_title_strip_width).
+  // 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
+  // -- there is then no title to measure, which is what keeps the two defaults
+  // above meaningful for the no-text-backend case. The x strip is one text line
+  // per titled x axis tall (they are stacked in it); the y strip is as wide as the
+  // widest y title, since a y title is a horizontal run that stacks down the strip
+  // and so needs width rather than another row of it (see y_title_strip_width).
+  const u32 x_titles = titled_x_axis_count();
   const f32 y_title_w = text_ != nullptr ? y_title_strip_width() : 0.0f;
-  const f32 x_title_h = text_ != nullptr && has_x_title() ? text_->height() + 2.0f * pad : 0.0f;
+  const f32 x_title_h = text_ != nullptr && x_titles != 0u
+                            ? static_cast<f32>(x_titles) * text_->height() + 2.0f * pad
+                            : 0.0f;
 
   for (u32 pass = 0;; ++pass) {
-    // Clamped to the viewport so a huge label set can never push the axes rect
-    // outside it (or make its width negative). The title strips take what is left
-    // after the label strip, so the two can never add up past the viewport.
+    // Chrome is measured against the viewport, then capped so the axes keeps
+    // kMinAxesShare of it -- the label strip first, the title strip out of what is
+    // left of the allowance -- which is what keeps axes.w and axes.h positive
+    // however long the labels and titles are. `viewport_` is what a huge label set
+    // is clamped to first, so neither strip can push the rect outside it.
     y_width = std::clamp(y_width, 0.0f, viewport_.w);
     x_height = std::clamp(x_height, 0.0f, viewport_.h);
-    const f32 y_title_width = std::clamp(y_title_w, 0.0f, viewport_.w - y_width);
-    const f32 x_title_height = std::clamp(x_title_h, 0.0f, viewport_.h - x_height);
+    const f32 y_budget = (1.0f - kMinAxesShare) * viewport_.w;
+    const f32 x_budget = (1.0f - kMinAxesShare) * viewport_.h;
+    y_width = std::min(y_width, y_budget);
+    x_height = std::min(x_height, x_budget);
+    const f32 y_title_width = std::min(y_title_w, y_budget - y_width);
+    const f32 x_title_height = std::min(x_title_h, x_budget - x_height);
     layout_.y_axis_width = y_width;
     layout_.x_axis_height = x_height;
     layout_.y_title_width = y_title_width;
@@ -344,8 +365,11 @@ const PlotLayout& Plot::layout() {
     refresh_ticks();
 
     if (text_ == nullptr || pass + 1 >= kMaxLayoutPasses) break;
-    const f32 need_y = std::clamp(y_label_width(), 0.0f, viewport_.w);
-    const f32 need_x = std::clamp(text_->height() + 2.0f * pad, 0.0f, viewport_.h);
+    // 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);
+    const f32 need_x =
+        std::min(std::clamp(text_->height() + 2.0f * pad, 0.0f, viewport_.h), x_budget);
     if (need_y == y_width && need_x == x_height) break;  // strips are a fixed point
     y_width = need_y;
     x_height = need_x;
@@ -385,18 +409,20 @@ f64 Plot::snapped_x(const Cursor& c) const {
   return c.x;
 }
 
-bool Plot::has_x_title() const noexcept {
+u32 Plot::titled_x_axis_count() const noexcept {
+  u32 n = 0;
   for (const Axis& a : x_axes_) {
-    if (!a.label.empty()) return true;
+    if (!a.label.empty()) ++n;
   }
-  return false;
+  return n;
 }
 
-bool Plot::has_y_title() const noexcept {
+u32 Plot::titled_y_axis_count() const noexcept {
+  u32 n = 0;
   for (const Axis& a : y_axes_) {
-    if (!a.label.empty()) return true;
+    if (!a.label.empty()) ++n;
   }
-  return false;
+  return n;
 }
 
 f32 Plot::y_title_strip_width() const {