code: def _onLeftButtonDClick(self, evt):
          
          x = evt.GetX()
          y = self.figure.bbox.height - evt.GetY()
          evt.Skip()
          self.CaptureMouse()
          FigureCanvasBase.button_press_event(self, x, y, 1, dblclick=True, guiEvent=evt)
docs: Start measuring on an axis.