feat(input): live x follow with relative zoom and mouse panning

include/plot/data/series.hpp

@@ -30,6 +30,15 @@ class Series {
   virtual Range x_bounds() const = 0;
   virtual Range y_bounds() const = 0;
   virtual const MinMaxPyramid& pyramid() const = 0;
+  /// True when the retained window slides as samples are produced, so that
+  /// `x_bounds()` reports the window the series currently holds rather than an
+  /// extent that only changes when the caller replaces the data. Only such a
+  /// series has a *live edge* for `XFollow::Live` to follow -- the newest x it
+  /// holds, which moves forward on its own -- so the follow path asks this rather
+  /// than guessing from a type: `StreamSeries` says true, and every other series
+  /// keeps the default. Virtual rather than a flag a caller could disagree with,
+  /// so the series that owns the sliding window is the one that answers.
+  virtual bool streaming() const noexcept { return false; }
 
   /// Appends interleaved (x, y) pairs in data coordinates for level `level`.
   ///