fix(plot): single-rounding clock labels, log-axis fits that refuse non-positive data

include/plot/plot/ticks.hpp

@@ -38,8 +38,18 @@ enum class TickFormat {
 void format_si(f64 v, char* buf, usize n, const char* unit = nullptr);
 
 /// Writes `seconds` as a compact clock string to `buf`, NUL-terminated and never
-/// past `n` bytes. Shapes: `500ms`, `1.5s`, `1:30`, `1:00:00`, `2.5d`; negatives
-/// keep their sign. A non-finite value falls back to `"%.3g"`.
+/// past `n` bytes. Negatives keep their sign; a non-finite value falls back to
+/// `"%.3g"`.
+///
+/// The whole value is rounded to a single whole-second count first and every
+/// field is derived from that one number, so no field can ever reach its radix:
+/// 119.5 s is `2:00`, not `1:60`, and 3599.5 s is `1:00:00`.
+///
+/// Shapes, by magnitude: below 1 s `500ms`; below 60 s `1.5s`; below 1 h `1:30`;
+/// at or above 24 h, as a decimal day count `2.5d`. Between 1 h and 24 h the
+/// shape is `1:01:01` -- and because a value that rounds up to 24 h takes the day
+/// shape, the hour field of that shape is always below 24. The 60 s and 1 h
+/// boundaries are inclusive of the rounded value: 59.5 s is `1:00`.
 void format_time(f64 seconds, char* buf, usize n);
 
 /// The hard cap on `generate_ticks` output, minor ticks included. A range dense