QwtPlot plotLayout().canvasRect() fails

Question

I want to plot figures with qwtplot like in Matlab with axis equal, so that a circle looks like a circle and not like an ellipse.

I have not found a function like "axis equal", or have missed one? So I want to write my own function. Therefore I need the size of the drawing area. I think "ployLayout().canvasRect()" should give me the size. When I use QtDesigner and uic, that works fine and I get the values, e.g:
PyQt4.QtCore.QRect(11, 0, 458, 412)

But if I programm the GUI on my own is does not work and the result is: PyQt4.QtCore.QRect()

This is my code

import sys
from PyQt4 import QtCore, QtGui, Qt
import PyQt4.Qwt5 as Qwt

class mainWindow(QtGui.QDialog):
    def __init__(self, numberOfObjects, parent=None):
        QtGui.QDialog.__init__(self, parent)

        okButton = QtGui.QPushButton(self.tr("OK"))
        cancelButton = QtGui.QPushButton(self.tr("Cancel"))

        self.connect(okButton, QtCore.SIGNAL("clicked()"), self, QtCore.SLOT("accept()"))
        self.connect(cancelButton, QtCore.SIGNAL("clicked()"), self, QtCore.SLOT("reject()"))

        self.testPlot =  Qwt.QwtPlot(self) 

        buttonLayout = QtGui.QHBoxLayout()
        buttonLayout.addStretch(1)
        buttonLayout.addWidget(okButton)
        buttonLayout.addWidget(cancelButton)

        mainLayout = QtGui.QVBoxLayout()
        mainLayout.addWidget(self.testPlot)
        mainLayout.addLayout(buttonLayout)

        self.setLayout(mainLayout)

        self.resize(450, 250)

        cr = self.testPlot.plotLayout().canvasRect()
        print cr


if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    win = mainWindow(0) 
    sys.exit(win.exec_())

Can anyone help me?
I use Python 2.7, QT4 and Qwt5.

Answer 1

Don't know about the Python bindings, but Qwt 5.2 offers a class QwtPlotRescaler, that offers something like the "axis equal" feature.

In Qwt 6.1 you find the rescaler example, that shows how to use it.