feat(plot): horizontal rulers alongside the vertical cursors

src/plot/axis.cpp

@@ -2,15 +2,24 @@
 
 namespace plot {
 
+void Axis::format_value(f64 v, char* buf, usize n) const {
+  if (format == TickFormat::Time) {
+    format_time(v, buf, n);
+    return;
+  }
+  format_si(v, buf, n);
+}
+
 void Axis::refresh(f32 px, f32 min_px, bool include_minors) {
   generate_ticks(range, px, min_px, ticks, include_minors);
   if (format != TickFormat::Time) return;
   // generate_ticks labels with format_si; a time axis relabels its majors here,
-  // because the tick generator has no business knowing about a clock.
+  // because the tick generator has no business knowing about a clock -- through the one
+  // function that says how a value on this axis is written.
   char buf[64];
   for (Tick& tick : ticks) {
     if (!tick.major) continue;
-    format_time(tick.value, buf, sizeof buf);
+    format_value(tick.value, buf, sizeof buf);
     tick.label = buf;
   }
 }