feat(plot): an axis can write its labels relative to an origin, and time scales by prefix

A host zooming into a sub-second window cannot label it absolutely: Time
formatting uses three significant digits, so 12.345671s and 12.345672s are the
same string and a 1 ms step around a multi-second time is unlabelled. The axis
gains an `origin`: every value it writes is `v - origin`, and the ticks are
generated on the shifted grid, so the window reads 0, 500us, 1ms.
`origin_follows_range` re-takes the origin from the axis' own range on every
refresh -- the mode a *moving* window needs, since the edge is derived from the
newest sample at layout time and a value the host wrote a frame earlier is a
frame of scroll stale.

`format_time` now scales its sub-second values by prefix, the way `format_si`
does on a y axis: the unit that keeps the mantissa in [1, 1000). A fixed
millisecond unit printed 0.005ms where 5us is the reading, and 1e-05ms where
10ns is. An exact zero prints `0`, the same convention `format_si` states. The
origin subtraction's dust is snapped by a *relative* epsilon, so a tick on the
origin reads 0 rather than 1.78e-12ms.

Tested by `ticks_are_written_relative_to_an_axis_origin`,
`a_range_origin_puts_the_zero_tick_on_the_left_edge` and the extended
`format_time_shapes`; the suite is 359 cases, 0 failed.

src/plot/axis.cpp

@@ -3,18 +3,32 @@
 
 #include <plot/plot/axis.hpp>
 
+#include <cmath>
+
 namespace plot {
 
 void Axis::format_value(f64 v, char* buf, usize n) const {
+  // The one place a value on this axis becomes text, so the origin is applied here and
+  // everything that goes through it -- tick labels, and the crosshair's readout -- reads in
+  // the same frame of reference as the axis itself.
+  // The subtraction leaves a residue when the value and the origin are the same distant number
+  // (a tick exactly on the origin then formatted as `1.78e-12ms` instead of `0ms`). Snapping it
+  // is a *relative* test, so a genuinely small value on a large axis is not swallowed -- what
+  // this removes is arithmetic dust, not data.
+  f64 shown = v - origin;
+  if (std::fabs(shown) < 1.0e-12 * std::max(1.0, std::fabs(v))) shown = 0.0;
   if (format == TickFormat::Time) {
-    format_time(v, buf, n);
+    format_time(shown, buf, n);
     return;
   }
-  format_si(v, buf, n);
+  format_si(shown, buf, n);
 }
 
 void Axis::refresh(f32 px, f32 min_px, bool include_minors) {
-  generate_ticks(range, px, min_px, ticks, include_minors);
+  // The range first, so a following origin is this frame's edge and not the last one's: the
+  // zero tick then sits exactly on the window start, which is the whole point of asking.
+  if (origin_follows_range) origin = range.min;
+  generate_ticks(range, px, min_px, ticks, include_minors, origin);
   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 -- through the one