How can I modify this code so that I am not replotting my graph everytime, matplotlib

Question

I am grabbing data from a device and wanting to plot the voltage of it, with the plot being embedded into the UI . I used the example here: http://matplotlib.org/examples/user_interfaces/embedding_in_qt4.html

This example works fine but when I add 2 or more graphs, the whole UI becomes really slow (using a RPi3) and CPU usage is really high. I realise this is probably because the graph is constantly being cleared and replotted.

My code looks like this:

class MyMplCanvas(FigureCanvas):
    def __init__(self, parent=None, width=5, height=2, dpi=100):
        fig = Figure(figsize=(width, height), dpi=dpi)
        self.axes = fig.add_subplot(111)

        self.compute_initial_figure()

        FigureCanvas.__init__(self, fig)
        self.setParent(parent)

        FigureCanvas.setSizePolicy(self,QtGui.QSizePolicy.Expanding,QtGui.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

    def compute_initial_figure(self):
        pass

class MyDynamicMplCanvas(MyMplCanvas):

    def __init__(self, *args, **kwargs):
        MyMplCanvas.__init__(self, *args, **kwargs)

    def compute_initial_figure(self):
        self.axes.cla()

    def update_figure(self,voltage):
        self.axes.cla()
        self.axes.plot(np.linspace(0,len(voltage)-1,num = len(voltage)), voltage, 'b')  
        self.axes.set_xlabel("Time")
        self.draw()


class worker_thread(QThread):
... #worker thread stuff here

class Monitor(QtGui.QMainWindow):

    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

        self.ui.exit_button.clicked.connect(exit)
        self.ui.go_button.clicked.connect(self.start_monitoring)
        self.ui.print_button.clicked.connect(self.test_print)

        self.ac_graph = QtGui.QWidget(self)
        self.ac_1_graph = MyDynamicMplCanvas(self.ac_graph,width = 10, height =3 , dpi = 60)
        self.ui.mplvl.addWidget(self.ac_1_graph)
        self.ac_1_graph.axes.set_xlabel("Time")

        self.dc_graph = QtGui.QWidget(self)
        self.dc_2_graph = MyDynamicMplCanvas(self.dc_graph,width = 10, height =3 , dpi = 60)
        self.ui.mplvl_2.addWidget(self.dc_2_graph)      

        self.ac1_voltage_values = []
        self.ac1_current_values = []
        self.dc2_voltage_values = []

    def start_monitoring(self): 

        self.worker_thread = worker_thread()

        self.connect(self.worker_thread,SIGNAL('grid_done'),      self.update_ac_dc_info)

    def update_plot_values(self, y_value, y_list):
        y_list.append(y_value)
        if (len(y_list) == 61):
            del y_list[0]
        return y_list

    def update_ac_dc_info(self,grid_info):
        self.ac1_voltage_values = self.update_plot_values((grid_info['ac1_voltage']/10),self.ac1_voltage_values)
        self.ac_1_graph.update_figure(self.ac1_voltage_values)

Essentially, when data comes back from my device, I emit a signal from the worker_thread which triggers my UI to update in the main thread, as well as the plot. At this point, how do I just get matplotlib to just accept the new point that came in WITHOUT replotting the whole thing? Many of the examples I've read utilise pyplot which I can't use because I need to embed this in an existing UI.

Answer 1

Instead of clearing the axes everytime there is new data, you can just redraw the line of points. This would save some time.

class MyDynamicMplCanvas(MyMplCanvas):

    def __init__(self, *args, **kwargs):
        MyMplCanvas.__init__(self, *args, **kwargs)

    def compute_initial_figure(self):
        # empty plot
        self.line,  = self.axes.plot([],[], color="b")
        self.axes.set_xlabel("Time") # xlabel will not change over time

    def update_figure(self,voltage):

        self.line.set_data(np.linspace(0,len(voltage)-1,num = len(voltage)), voltage)  
        # now you need to take care of the axis limits yourself
        self.axes.set_xlim([0,len(voltage])
        self.draw()

Updating only the last new point is a bit tricky, since the plot consist of a single Line2D object. You could opt for a point plot, where you can plot a single new point every time new data arrives.

Answer 2

ImportanceOfBeingErnest's method works just fine. To rescale the axis based on the data:

self.axes.relim()
self.axes.autoscale_view(True,True,True)