如何让FigureCanvas在pyqt GUI中嵌入的matplotlib小部件中填充整个图形？

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QWidget
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure

class MyWidget(QWidget):
    def __init__(self, parent=None):
        super(MyWidget, self).__init__(parent)
        self.figure = Figure()
        self.canvas = FigureCanvas(self.figure)
        layout = QVBoxLayout()
        layout.addWidget(self.canvas)
        self.setLayout(layout)

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setWindowTitle("Matplotlib in PyQt")
        self.setGeometry(100, 100, 800, 600)
        self.widget = MyWidget(self)
        self.setCentralWidget(self.widget)

def plot_graph():
    ax = widget.figure.add_subplot(111)
    ax.plot([1, 2, 3, 4, 5], [1, 4, 9, 16, 25])
    widget.canvas.draw()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    plot_graph()
    sys.exit(app.exec_())