Plot custom zoom (#2714)

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- Added methods to zoom the plot programmatically, to match the
previously added `translate_bounds()`.
- Added an example of how this method can be used to customize the plot
navigation.

Closes #1164.

crates/egui_plot/src/lib.rs

@@ -1002,6 +1002,10 @@ impl Plot {
                     auto_bounds = false.into();
                 }
                 BoundsModification::AutoBounds(new_auto_bounds) => auto_bounds = new_auto_bounds,
+                BoundsModification::Zoom(zoom_factor, center) => {
+                    bounds.zoom(zoom_factor, center);
+                    auto_bounds = false.into();
+                }
             }
         }
 
@@ -1341,6 +1345,7 @@ enum BoundsModification {
     Set(PlotBounds),
     Translate(Vec2),
     AutoBounds(Vec2b),
+    Zoom(Vec2, PlotPoint),
 }
 
 /// Provides methods to interact with a plot while building it. It is the single argument of the closure
@@ -1404,6 +1409,31 @@ impl PlotUi {
         &self.response
     }
 
+    /// Scale the plot bounds around a position in screen coordinates.
+    ///
+    /// Can be useful for implementing alternative plot navigation methods.
+    ///
+    /// The plot bounds are divided by `zoom_factor`, therefore:
+    /// - `zoom_factor < 1.0` zooms out, i.e., increases the visible range to show more data.
+    /// - `zoom_factor > 1.0` zooms in, i.e., reduces the visible range to show more detail.
+    pub fn zoom_bounds(&mut self, zoom_factor: Vec2, center: PlotPoint) {
+        self.bounds_modifications
+            .push(BoundsModification::Zoom(zoom_factor, center));
+    }
+
+    /// Scale the plot bounds around the hovered position, if any.
+    ///
+    /// Can be useful for implementing alternative plot navigation methods.
+    ///
+    /// The plot bounds are divided by `zoom_factor`, therefore:
+    /// - `zoom_factor < 1.0` zooms out, i.e., increases the visible range to show more data.
+    /// - `zoom_factor > 1.0` zooms in, i.e., reduces the visible range to show more detail.
+    pub fn zoom_bounds_around_hovered(&mut self, zoom_factor: Vec2) {
+        if let Some(hover_pos) = self.pointer_coordinate() {
+            self.zoom_bounds(zoom_factor, hover_pos);
+        }
+    }
+
     /// The pointer position in plot coordinates. Independent of whether the pointer is in the plot area.
     pub fn pointer_coordinate(&self) -> Option<PlotPoint> {
         // We need to subtract the drag delta to keep in sync with the frame-delayed screen transform:

crates/egui_plot/src/transform.rs

@@ -150,6 +150,13 @@ impl PlotBounds {
         self.translate_y(delta.y as f64);
     }
 
+    pub(crate) fn zoom(&mut self, zoom_factor: Vec2, center: PlotPoint) {
+        self.min[0] = center.x + (self.min[0] - center.x) / (zoom_factor.x as f64);
+        self.max[0] = center.x + (self.max[0] - center.x) / (zoom_factor.x as f64);
+        self.min[1] = center.y + (self.min[1] - center.y) / (zoom_factor.y as f64);
+        self.max[1] = center.y + (self.max[1] - center.y) / (zoom_factor.y as f64);
+    }
+
     pub(crate) fn add_relative_margin_x(&mut self, margin_fraction: Vec2) {
         let width = self.width().max(0.0);
         self.expand_x(margin_fraction.x as f64 * width);
@@ -255,10 +262,7 @@ impl PlotTransform {
         let center = self.value_from_position(center);
 
         let mut new_bounds = self.bounds;
-        new_bounds.min[0] = center.x + (new_bounds.min[0] - center.x) / (zoom_factor.x as f64);
-        new_bounds.max[0] = center.x + (new_bounds.max[0] - center.x) / (zoom_factor.x as f64);
-        new_bounds.min[1] = center.y + (new_bounds.min[1] - center.y) / (zoom_factor.y as f64);
-        new_bounds.max[1] = center.y + (new_bounds.max[1] - center.y) / (zoom_factor.y as f64);
+        new_bounds.zoom(zoom_factor, center);
 
         if new_bounds.is_valid() {
             self.bounds = new_bounds;