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

docs/superpowers/plans/2026-09-27-plot-library-scaffold.md

@@ -1019,17 +1019,23 @@ Expected: FAIL to compile — `plot/plot/ticks.hpp` not found.
 `src/plot/ticks.cpp` — the nice-number algorithm:
 
 ```cpp
-// generate_ticks(range, px, min_px, out):
-//   if (!(range.span() > 0) || px <= 0) { out.clear(); return; }
+// generate_ticks(range, px, min_px, out, bool include_minors = false):
+//   if (!(range.span() > 0) || px <= 0 || !isfinite(span) || min > max) { out.clear(); return; }
 //   const f64 target_count = std::max(1.0, f64(px) / f64(min_px));
 //   const f64 raw_step = range.span() / target_count;
 //   const f64 mag  = std::pow(10.0, std::floor(std::log10(raw_step)));
 //   const f64 norm = raw_step / mag;                       // in [1, 10)
-//   const f64 step = mag * (norm <= 1.5 ? 1.0 : norm <= 3.0 ? 2.0 : norm <= 7.0 ? 5.0 : 10.0);
+//   // ROUND UP, not to nearest. Rounding to nearest can exceed target_count and
+//   // break the "at most px/min_px + 2 ticks" bound that the tests assert.
+//   const f64 step = mag * (norm <= 1.0 ? 1.0 : norm <= 2.0 ? 2.0 : norm <= 5.0 ? 5.0 : 10.0);
 //   f64 first = std::ceil(range.min / step) * step;
 //   for (f64 v = first; v <= range.max + step * 1e-9; v += step) out.push_back({v, true, {}});
-//   // minor ticks: step/5 (or step/4 when step starts with 2, step/5 when with 5), same loop.
-//   // hard guard: if (out.size() > 256) out.resize(256);
+//   // include_minors defaults to false because minors share this same vector: a
+//   // caller that wants only majors must not have to filter, and the "every value
+//   // is a multiple of the major step" assertions only hold for the majors-only
+//   // form. Grid rendering passes true.
+//   // minor ticks: step/5 (step/4 for a step beginning with 2), same loop.
+//   // hard guard: 256 entries maximum INCLUDING minors.
 ```
 Labels are assigned by `format_si` unless the axis is a time axis, in which case `format_time` is used. Ticks whose label would be non-finite or absurdly long (> 12 chars) fall back to `%.3g`.