'Start measuring on an axis.'

def _onLeftButtonDClick(self, evt):

    x = evt.GetX()
    y = (self.figure.bbox.height - evt.GetY())
    evt.Skip()
    self._set_capture(True)
    FigureCanvasBase.button_press_event(self, x, y, 1, dblclick=True, guiEvent=evt)
