def on_key_press(self, event):
        """Pan and zoom with the keyboard."""
        # Zooming with the keyboard.
        key = event.key
        if event.modifiers:
            return

        # Pan.
        if self.enable_keyboard_pan and key in self._arrows:
            self._pan_keyboard(key)

        # Zoom.
        if key in self._pm:
            self._zoom_keyboard(key)

        # Reset with 'R'.
        if key == 'R':
            self.reset()
Pan and zoom with the keyboard.