called when x limits are changed

def xlim_changed(self, axsubplot):
        '''called when x limits are changed'''
        xrange = axsubplot.get_xbound()
        xlim = axsubplot.get_xlim()
        self.setup_xrange(xrange[1] - xrange[0])
        if self.draw_events == 0:
            # ignore limit change before first draw event
            return
        if self.xlim_pipe is not None and axsubplot == self.ax1 and xlim != self.xlim:
            self.xlim = xlim
            #print('send', self.graph_num, xlim)
            self.xlim_pipe[1].send(xlim)