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

include/plot/plot/plot.hpp

@@ -109,6 +109,13 @@ class Plot {
   /// Fits every x axis to the union of the series bounds of the traces bound to
   /// it, padded by 2% of the span, and re-arms auto scale on the axes it fitted.
   /// An axis with no visible trace is left exactly as it was.
+  ///
+  /// A log axis is fitted in log space and needs a positive value to show. When
+  /// its traces hold none (every sample negative or zero) the fit is refused: the
+  /// axis keeps its current range, auto scale is not re-armed, and the next
+  /// auto-fit tries again -- so a stream that starts at zero begins fitting on its
+  /// own once it goes positive. A log axis is therefore never left with `min >=
+  /// max` or a non-positive bound by an auto-fit.
   void auto_fit_x();
   /// Same for one y axis, over that axis' traces' y bounds.
   void auto_fit_y(u32 axis);
@@ -161,10 +168,15 @@ class Plot {
     mutable u64 last_size = 0;
   };
 
-  /// Fit one axis to its traces' bounds, leaving auto_scale alone. False when no
-  /// visible trace is bound to the axis, in which case the range is untouched.
+  /// Fit one axis to its traces' bounds, leaving auto_scale alone. False when the
+  /// axis cannot be fitted (no visible trace bound to it, or a log axis whose
+  /// traces hold no positive value), in which case the range is untouched.
   bool fit_x_axis(u32 axis);
   bool fit_y_axis(u32 axis);
+  /// Union of the bounds of the visible traces bound to one axis (`x_side`
+  /// selects x or y). False when there is no such trace; `out` then retains
+  /// whatever it held.
+  bool trace_bounds_union(bool x_side, u32 axis, Range& out) const;
   /// Refit every axis that is still on auto scale. layout() runs this, so a
   /// growing stream stays framed until a caller gesture turns auto scale off.
   void autoscale_axes();