Modifying Matplotlib figure axes within PyQT4

Question

I would like to create an application that embeds a Matplotlib animation within a PyQT4 GUI. I am trying to figure out the basics of how a FigureCanvasQTAgg object works, and am having trouble changing the axis limits once it is created. In the program below, a very simple figure is generated using the FigureCanvasQTAgg object, and I would like the press of the button to change the limits of the x-axis of the resulting plot. (Note that the following code is a simplified version of the code at the end of this post .)

import sys
from PyQt4 import QtGui
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure

class Window(QtGui.QDialog): 

def __init__(self):
    super(Window, self).__init__()
    self.fig = Figure()
    self.canvas = FigureCanvas(self.fig)
    self.ax = self.fig.add_subplot(111)  # create an axis
    self.ax.hold(False)  # discards the old graph
    self.ax.set_xlim([0, 100])
    self.ax.set_ylim([-10, 10])
    self.ax.set_xlabel('Random x label')
    self.button = QtGui.QPushButton('Adjust axes')
    self.button.clicked.connect(self.axis_adjust)

    # set the layout
    layout = QtGui.QVBoxLayout()
    layout.addWidget(self.canvas)
    layout.addWidget(self.button)
    self.setLayout(layout)
    self.canvas.draw()

def axis_adjust(self):
    self.ax.set_xlim([0, 200])
    self.ax.set_xlabel('New label')


app = QtGui.QApplication(sys.argv)
ex = Window()
sys.exit(app.exec_())

Unfortunately, when I run my program and click the "Adjust axes" button, it has absolutely no effect. One thing I fundamentally do not understand is what the line self.canvas.draw() does. From a Matplotlib tutorial , I got the impression that this makes the figure actually appear within the GUI window...but this is not the case, because when I comment out the line self.canvas.draw() , the figure still appears within the GUI window. It is actually the line self.setLayout(layout) that makes the figure appear in the GUI window, which does not make sense from what I have read. It seems my lack of understanding of this functionality is at the root of my problem...

Answer 1

I wrote a simple application before. In my experience, self.canvas.draw() must be called after updating the settings or plotting a curve ( or any other object) using matplotlib functionalities. It is simply a function to trigger the update of any graphical objects. So just add self.canvas.draw() to the end of your callback function axis_adjust() should fix your problem.

FYI, the hierarchy of your UI will be QtGui.QDialog -> QtGui.QVBoxLayout -> FigureCanvas -> matplotlib.figure.Figure