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

include/plot/data/stream_series.hpp

@@ -29,7 +29,8 @@ namespace plot::data {
 /// base, so that acquire re-reads the whole window.
 ///
 /// x0() is therefore the x of the oldest retained sample, x_bounds().min == x0(),
-/// and y_bounds() covers exactly the published samples.
+/// and y_bounds() covers exactly the published samples. `streaming()` says true:
+/// this is the window that slides, the one `XFollow::Live` follows.
 ///
 /// THREADING NOTE. The ring overwrites the oldest samples -- that is what keeps a
 /// live stream showing the newest data, and what the drop test pins down -- so a
@@ -72,6 +73,9 @@ class StreamSeries final : public Series {
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
   void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy) const override;
+  /// Always true: this series' `x_bounds()` is the retained ring window, which
+  /// slides with every `acquire()` -- the property `XFollow::Live` follows.
+  bool streaming() const noexcept override { return true; }
 
  protected:
   void copy_raw(u64 begin, u64 end, f32* out) const override;