ENH: Scroll to zoom (#30405)

timhoffm · QuLogic · web-flow · commit f2a768287bef · 2025-09-10T10:01:14.000+02:00
* ENH: Scroll to zoom Implements a minimal version of #20317, in particular https://github .com//pull/20317#issuecomment-2233156558: Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>
diff --git a/doc/release/next_whats_new/scroll_to_zoom.rst b/doc/release/next_whats_new/scroll_to_zoom.rst
@@ -0,0 +1,8 @@
+Zooming using mouse wheel
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``Ctrl+MouseWheel`` can be used to zoom in the plot windows.
+
+The zoom focusses on the mouse pointer, and keeps the aspect ratio of the axes.
+
+Zooming is currently only supported on rectilinear Axes.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -2574,6 +2574,51 @@ def button_press_handler(event, canvas=None, toolbar=None):
             toolbar.forward()
 
 
+def scroll_handler(event, canvas=None, toolbar=None):
+    ax = event.inaxes
+    if ax is None:
+        return
+    if ax.name != "rectilinear":
+        # zooming is currently only supported on rectilinear axes
+        return
+
+    if toolbar is None:
+        toolbar = (canvas or event.canvas).toolbar
+
+    if toolbar is None:
+        # technically we do not need a toolbar, but until wheel zoom was
+        # introduced, any interactive modification was only possible through
+        # the toolbar tools. For now, we keep the restriction that a toolbar
+        # is required for interactive navigation.
+        return
+
+    if event.key == "control":  # zoom towards the mouse position
+        toolbar.push_current()
+
+        xmin, xmax = ax.get_xlim()
+        ymin, ymax = ax.get_ylim()
+        (xmin, ymin), (xmax, ymax) = ax.transScale.transform(
+            [(xmin, ymin), (xmax, ymax)])
+
+        # mouse position in scaled (e.g., log) data coordinates
+        x, y = ax.transScale.transform((event.xdata, event.ydata))
+
+        scale_factor = 0.85 ** event.step
+        new_xmin = x - (x - xmin) * scale_factor
+        new_xmax = x + (xmax - x) * scale_factor
+        new_ymin = y - (y - ymin) * scale_factor
+        new_ymax = y + (ymax - y) * scale_factor
+
+        inv_scale = ax.transScale.inverted()
+        (new_xmin, new_ymin), (new_xmax, new_ymax) = inv_scale.transform(
+            [(new_xmin, new_ymin), (new_xmax, new_ymax)])
+
+        ax.set_xlim(new_xmin, new_xmax)
+        ax.set_ylim(new_ymin, new_ymax)
+
+        ax.figure.canvas.draw_idle()
+
+
 class NonGuiException(Exception):
     """Raised when trying show a figure in a non-GUI backend."""
     pass
@@ -2653,11 +2698,14 @@ def __init__(self, canvas, num):
 
         self.key_press_handler_id = None
         self.button_press_handler_id = None
+        self.scroll_handler_id = None
         if rcParams['toolbar'] != 'toolmanager':
             self.key_press_handler_id = self.canvas.mpl_connect(
                 'key_press_event', key_press_handler)
             self.button_press_handler_id = self.canvas.mpl_connect(
                 'button_press_event', button_press_handler)
+            self.scroll_handler_id = self.canvas.mpl_connect(
+                'scroll_event', scroll_handler)
 
         self.toolmanager = (ToolManager(canvas.figure)
                             if mpl.rcParams['toolbar'] == 'toolmanager'
diff --git a/lib/matplotlib/backend_bases.pyi b/lib/matplotlib/backend_bases.pyi
@@ -407,6 +407,11 @@ def button_press_handler(
     canvas: FigureCanvasBase | None = ...,
     toolbar: NavigationToolbar2 | None = ...,
 ) -> None: ...
+def scroll_handler(
+    event: MouseEvent,
+    canvas: FigureCanvasBase | None = ...,
+    toolbar: NavigationToolbar2 | None = ...,
+) -> None: ...
 
 class NonGuiException(Exception): ...
 
@@ -415,6 +420,7 @@ class FigureManagerBase:
     num: int | str
     key_press_handler_id: int | None
     button_press_handler_id: int | None
+    scroll_handler_id: int | None
     toolmanager: ToolManager | None
     toolbar: NavigationToolbar2 | ToolContainerBase | None
     def __init__(self, canvas: FigureCanvasBase, num: int | str) -> None: ...
