Matplotlib embedded PyQt4 graph resizes every time I plot

Question

I have an embedded matplotlib plot in a PyQt4 application. This is the plotting widget I have made based on the Qt4 embedding sample at http://matplotlib.sourceforge.net/examples/user_interfaces/embedding_in_qt4.html. Unfortunately, every time I add a new plot, this graph shrinks slightly. I have two plot widgets aligned side by side in a QGridLayout if that makes a difference.

What could the reason for this weird behaviour be?

from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure

class MultiPlot(FigureCanvas):
    def __init__(self, parent=None, width=5, height=4, dpi=100):
        fig = Figure(figsize=(width, height), dpi=dpi)
        self.axes = fig.add_subplot(111)
        FigureCanvas.__init__(self, fig)
        self.setParent(parent)
        self.setMinimumSize(300, 200)
        self.setMaximumSize(600, 400)

    def plot(self,x,y):
        print "Plot"
        self.axes.plot(x,y)
        FigureCanvas.updateGeometry(self)

    def changePlot(self,plotNum,x,y):
        print "Change plot",plotNum
        self.axes.lines[plotNum].set_data(x,y)
        FigureCanvas.updateGeometry(self)

Answer 1

I fixed this using setFixedSize on the widget instead of allowing the layout manager to resize. I dont understand why the layout manager would want to shrink the widget when more plots were added but the problem is fixed anyway