Start measuring on an axis.

def _onLeftButtonDClick(self, evt):
        """Start measuring on an axis."""
        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)