WIP: Make + and - zoom in and out, respectively

This adds a zoom function to the PCanvas and implements for the two
current backends. For vispy, the zoom happens under the cursor, like how
the mouse wheel behaves. But for pygfx, I did not yet figure out how to
do that. Do we need to translate the focused point to the origin, then
change zoom, then translate it back?

src/ndv/viewer/_backends/_protocols.py

@@ -97,6 +97,11 @@ def set_range(
         z: tuple[float, float] | None = None,
         margin: float = ...,
     ) -> None: ...
+    def zoom(
+        self,
+        factor: Union[float, tuple],
+        center: tuple = None,
+    ) -> None: ...
     def refresh(self) -> None: ...
     def qwidget(self) -> QWidget: ...
     def add_image(

src/ndv/viewer/_backends/_pygfx.py

@@ -561,6 +561,23 @@ def set_range(
         cam.zoom = 1 - margin
         self.refresh()
 
+    def zoom(self, factor: Union[float, tuple], center: tuple = None):
+        """
+        Zoom in (or out) at the given center.
+
+        Parameters
+        ----------
+        factor : float or tuple
+            Fraction by which the scene should be zoomed (e.g. a factor of 2
+            causes the scene to appear twice as large).
+        center : tuple of 2-4 elements
+            The center of the view. If not given or None, use the
+            current center.
+        """
+        cam = self._camera
+        cam.zoom *= factor
+        self.refresh()
+
     def refresh(self) -> None:
         self._canvas.update()
         self._canvas.request_draw(self._animate)

src/ndv/viewer/_backends/_vispy.py

@@ -2,7 +2,7 @@
 
 import warnings
 from contextlib import suppress
-from typing import TYPE_CHECKING, Any, Literal, cast
+from typing import TYPE_CHECKING, Any, Literal, Union, cast
 from weakref import WeakKeyDictionary
 
 import cmap
@@ -580,6 +580,21 @@ def set_range(
             max_size = max(x[1], y[1], z[1])
             self._camera.scale_factor = max_size + 6
 
+    def zoom(self, factor: Union[float, tuple], center: tuple = None):
+        """
+        Zoom in (or out) at the given center.
+
+        Parameters
+        ----------
+        factor : float or tuple
+            Fraction by which the scene should be zoomed (e.g. a factor of 2
+            causes the scene to appear twice as large).
+        center : tuple of 2-4 elements
+            The center of the view. If not given or None, use the
+            current center.
+        """
+        return self._camera.zoom(factor=factor, center=center)
+
     def canvas_to_world(
         self, pos_xy: tuple[float, float]
     ) -> tuple[float, float, float]:

src/ndv/viewer/_viewer.py

@@ -153,6 +153,9 @@ def __init__(
         # ROI
         self._roi: PRoiHandle | None = None
 
+        # closest data point under the mouse
+        self._data_coords: tuple[int, int] = (0, 0)
+
         # WIDGETS ----------------------------------------------------
 
         # the button that controls the display mode of the channels
@@ -731,6 +734,7 @@ def _update_hover_info(self, event: QMouseEvent) -> bool:
 
         x = int(x)
         y = int(y)
+        self._data_coords = (x, y)
         text = f"[{y}, {x}]"
         # TODO: Can we use self._canvas.elements_at?
         for n, handles in enumerate(self._img_handles.values()):
@@ -765,6 +769,10 @@ def keyPressEvent(self, a0: QKeyEvent | None) -> None:
         if a0.key() == Qt.Key.Key_Delete and self._selection is not None:
             self._selection.remove()
             self._selection = None
+        elif a0.key() in [Qt.Key.Key_Plus, Qt.Key.Key_Equal]:
+            self._canvas.zoom(factor=0.667, center=self._data_coords)
+        elif a0.key() in [Qt.Key.Key_Minus, Qt.Key.Key_Underscore]:
+            self._canvas.zoom(factor=1.5, center=self._data_coords)
 
     def _update_roi_button(self, event: QMouseEvent) -> bool:
         if self._add_roi_btn.isChecked():