code: def on_mouse_wheel(self, event):
          
          # NOTE: not called on OS X because of touchpad
          if event.modifiers:
              return
          dx = np.sign(event.delta[1]) * self._wheel_coeff
          # Zoom toward the mouse pointer.
          x0, y0 = self._normalize(event.pos)
          self.zoom_delta((dx, dx), (x0, y0))
docs: Zoom with the mouse wheel.