feat(plot): a figure can keep an empty band to the right of its data

`layout_.axes` reached the viewport's right edge, so the rightmost x tick
label -- centred on its tick -- had half of itself outside the plot, and
the newest sample's ink landed on the frame. `Plot::set_right_gutter()`
reserves a band there, out of the same `1 - kMinAxesShare` allowance the
label and title strips are served from, so it can no more squeeze `axes`
below the half-viewport floor than they can. `Figure` sets one value on
every row, the left gutter's reason: the rows' axes have to end on a
common column as well as start on one.

Nothing is drawn in the band, so unlike the strips it is a want and never
a measurement: no measuring pass, and it cannot feed the layout's fixed
point.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

include/plot/plot/plot.hpp

@@ -88,6 +88,11 @@ struct PlotLayout {
   /// no text backend there is no title to lay out at all and both stay 0.0f.
   f32 y_title_width = 0.0f;
   f32 x_title_height = 0.0f;
+  /// The empty band kept between `axes`' right edge and the viewport's, 0.0f unless a
+  /// caller asked for one (see `Plot::set_right_gutter()`). Reported because it is
+  /// clamped by the same cap as the strips above, so what was asked for and what was
+  /// reserved are not always the same number.
+  f32 right_gutter = 0.0f;
   /// The measured legend: inside `axes`, never reserving any part of it, and empty
   /// when there is nothing to draw. It is measured with the same text backend the
   /// strips above are, and against the same `axes` rect the caller reads back, so
@@ -640,6 +645,20 @@ class Plot {
   }
   f32 min_y_gutter() const noexcept { return min_y_gutter_; }
 
+  /// An empty band reserved between `axes`' right edge and the viewport's, in device
+  /// pixels. 0.0 -- the default -- is what the layout has always done: `axes` reaches
+  /// the viewport's right edge, which leaves the rightmost x tick label, *centred* on
+  /// that edge, half of it to be drawn in and nothing to draw it in -- and puts the
+  /// newest sample's ink on the frame. It is the mirror of the left gutter in purpose
+  /// and not in kind: nothing is drawn here, so there is nothing to measure and the
+  /// value is the caller's to choose. Reserved inside the same cap as the label and
+  /// title strips, so it can no more squeeze `axes` below the half-viewport floor than
+  /// they can. Negative values are clamped to 0.
+  void set_right_gutter(f32 device_px) noexcept {
+    right_gutter_ = device_px > 0.0f ? device_px : 0.0f;
+  }
+  f32 right_gutter() const noexcept { return right_gutter_; }
+
   /// Grid segments for the primary axis pair (x axis 0, y axis 0), in data space.
   const Grid& grid() const noexcept { return grid_; }
 
@@ -867,6 +886,9 @@ class Plot {
   /// The floor a figure imposes on the left gutter; see set_min_y_gutter(). 0.0 means
   /// none, which is what a plot standing on its own keeps.
   f32 min_y_gutter_ = 0.0f;
+  /// The empty band kept to the right of `axes`; see set_right_gutter(). 0.0 means none,
+  /// which is what the layout did before the setter existed.
+  f32 right_gutter_ = 0.0f;
   /// Which handles a pointer is on, published by the input layer (see HotHandles).
   HotHandles hot_{};