how to force matplotlib to update a plot

Question

I am trying to construct a little GUI that has a plot which updates every time a new data sample is read. I would prefer not to run it with a timer, since the data will be arriving at differing intervals. Instead, I'm trying to make an implementation using signals, where the data collection function will emit a signal when data is read, and then the painting function will emit a signal when the painting is completed.

The problem, as it appears right now, is that the canvas is not updating as soon as I call canvas.draw(). When this program runs, data_collect() and paint() alternate sending signals, but the figure is not updated until after I stop the process. How can I force matplotlib to update the figure whenever paint() is called?

What follows is a relatively simple piece of example code which is not optimal, but hopefully will convey the flavor of what I'm trying to do...

N_length = 150;
count = [0];

def sinval(delay):

    k = 0;
    x = [];

    # set up data vector with sinusoidal data in it.
    while k < N_length:  
        x.append(math.sin(2*math.pi*k/N_length));
        k += 1;

    def next():

        time.sleep(delay);
        outstring = "%0.3e" % (x[count[0]]);

        if (count[0] == (N_length-1)):
            count[0] = 0;
        else:
            count[0] += 1;

        return outstring;

    return next;


class DesignerMainWindow(QtGui.QMainWindow, Ui_mplMainWindow):

    def __init__(self, parent = None):
        super(DesignerMainWindow, self).__init__(parent)
        self.setupUi(self)

        QtCore.QObject.connect(self.mplStartButton, QtCore.SIGNAL("clicked()"), self.start_graph);
        QtCore.QObject.connect(self.mplStopButton, QtCore.SIGNAL("clicked()"), self.stop_graph);
        QtCore.QObject.connect(self.mplQuitButton, QtCore.SIGNAL("clicked()"), QtGui.qApp, QtCore.SLOT("quit()"));
        QtCore.QObject.connect(self, QtCore.SIGNAL("data_collect()"), self.data_collect);
        QtCore.QObject.connect(self, QtCore.SIGNAL("paint()"), self.paint);

    def start_graph(self):

        # generates first "empty" plots
        self.user = [];
        self.l_user, = self.mpl.canvas.ax.plot([], self.user, label='sine wave');

        # set up the axes. 
        self.mpl.canvas.ax.set_xlim(0, 300);
        self.mpl.canvas.ax.set_ylim(-1.1, 1.1);
        self.mpl.canvas.draw();

        # start the data collection process.
        self.delay = 0.05;
        self.next = sinval(self.delay);
        self.emit(QtCore.SIGNAL('data_collect()'));


    def data_collect(self):
        outstring = self.next();
        self.user.append(float(outstring.split()[0]));
        self.l_user.set_data(range(len(self.user)), self.user);
        self.emit(QtCore.SIGNAL('paint()'));


    def paint(self):
        self.mpl.canvas.draw();
        self.emit(QtCore.SIGNAL('data_collect()'));

Answer 1

I'd guess that calling QCoreApplication::processEvents after paint() will help. More elegant would be to have a separate QThread for the reading. Take a look at this thread.