handle xlim change requests from queue

def xlim_change_check(self, idx):
        '''handle xlim change requests from queue'''
        if not self.xlim_pipe[1].poll():
            return
        xlim = self.xlim_pipe[1].recv()
        if xlim is None:
            return
        #print("recv: ", self.graph_num, xlim)
        if self.ax1 is not None and xlim != self.xlim:
            self.xlim = xlim
            self.fig.canvas.toolbar.push_current()
            #print("setting: ", self.graph_num, xlim)
            self.ax1.set_xlim(xlim)
            # trigger the timer, this allows us to setup a v slow animation,
            # which saves a lot of CPU
            self.ani.event_source._on_timer()