# "manual fork" of https://gist.github.com/iMichka/9901318

MPL_blitting.py

#!/usr/bin/env python

# Original source : https://gist.github.com/iMichka/9901318
# Change : use a timer

import sys
import numpy
from PyQt4 import QtGui, QtCore
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
from matplotlib.collections import PolyCollection
from matplotlib.figure import Figure


class MainWindow(QtGui.QMainWindow):

    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.setSizePolicy(QtGui.QSizePolicy.Minimum, QtGui.QSizePolicy.Minimum)

        subw = SubWidget()
        tabs = QtGui.QTabWidget()
        tabs.addTab(subw, "Data")

        self.setCentralWidget(tabs)


class SubWidget(QtGui.QWidget):

    def __init__(self):
        super(QtGui.QWidget, self).__init__()

        VL = QtGui.QVBoxLayout()

        mesh_size = [300, 300]
        canvas = QtGui.QWidget()
        sizes = [mesh_size[0]/72.0, mesh_size[1]/72.0, 72.0]
        self.myplot = MyPlot(self, canvas, sizes)
        self.myplot.mpl_connect("button_press_event", self.mesh_press)

        VL.addWidget(self.myplot)

        self.setLayout(VL)
        #self.init_plot() 
        # Init the plot after Gui is set up
        self.timer  = QtCore.QTimer.singleShot(0, self.init_plot)
    
    def init_plot(self):
        self.myplot.update_plot()
        self.myplot.update_blit()
    
    def mesh_press(self, event):

        xpos, ypos = int(event.xdata), int(event.ydata)
        self.myplot.xpos = xpos
        self.myplot.ypos = ypos

        self.myplot.update_blit()

class MyPlot(FigureCanvasQTAgg):

    def __init__(self, parent, canvas, sizes):

        self.parent = parent
        self.canvas = canvas

        self.xpos = 0
        self.ypos = 0

        self.fig = Figure(figsize = (sizes[0], sizes[1]), dpi = sizes[2])

        FigureCanvasQTAgg.__init__(self, self.fig)

        FigureCanvasQTAgg.setSizePolicy(self,
            QtGui.QSizePolicy.MinimumExpanding,
            QtGui.QSizePolicy.MinimumExpanding)
        FigureCanvasQTAgg.updateGeometry(self)
        self.setParent(canvas)


    def update_plot(self):

        self.fig.clf()
        self.axes = self.fig.add_subplot(111)

        array = numpy.random.randint(10, size=(20, 20))
        self.axes.pcolormesh(array, cmap = "copper")

        # Draw in the canvas
        self.fig.canvas.draw()

        self.empty_bbox = self.copy_from_bbox(self.axes.bbox)


    def update_blit(self):

        self.restore_region(self.empty_bbox)
        self.draw_red_square()
        self.blit(self.axes.bbox)


    def draw_red_square(self):

        x_size = 1
        y_size = 1

        pos = [self.xpos, self.ypos]
        x1 = pos[0]*x_size + x_size
        y1 = pos[1]*y_size + y_size
        x2 = pos[0]*x_size + x_size
        y2 = pos[1]*y_size
        x3 = pos[0]*x_size
        y3 = pos[1]*y_size
        x4 = pos[0]*x_size
        y4 = pos[1]*y_size + y_size

        verts = numpy.array([(x1, y1), (x2, y2), (x3, y3), (x4, y4)])
        coll = PolyCollection([verts], facecolors = "none",
                                       edgecolors = "r", linewidths = 2)
        self.axes.add_collection(coll)

        self.axes.draw_artist(coll)


def main():

    app = QtGui.QApplication(["Test"])

    main_window = MainWindow()
    main_window.show()
    if sys.platform == "darwin":
        main_window.raise_()

    app.exec_()


if __name__ == "__main__":

    main()