def _onLeftButtonDown(self, evt):
x = evt.GetX()<EOL>y = self.figure.bbox.height - evt.GetY()<EOL>evt.Skip()<EOL>self.CaptureMouse()<EOL>FigureCanvasBase.button_press_event(self, x, y, <NUM_LIT:1>, guiEvent=evt)<EOL>
Start measuring on an axis.