Place rectangle in drawing

For this question I am referring to this example : https://matplotlib.org/examples/user_interfaces/embedding_in_qt5.html

I would like to replace the sinogram with a simple rectangle plot that doesn't show the coordinate system .

To be precise, I want to change this part of the original code:

class MyStaticMplCanvas(MyMplCanvas):
    """Simple canvas with a sine plot."""

    def compute_initial_figure(self):
        t = arange(0.0, 3.0, 0.01)
        s = sin(2*pi*t)
        self.axes.plot(t, s)

I found the rectangle class in Matplotlib : https://matplotlib.org/devdocs/api/_as_gen/matplotlib.patches.Rectangle.html#examples-using-matplotlib-patches-rectangle

This should be an easy task. But I've tried a lot myself without success. One of my many attempts looks like: (but it doesn't work)

class MyStaticMplCanvas(MyMplCanvas):
    """Simple canvas with a rectangle plot."""
    def compute_initial_figure(self):
        x = 10
        y = 20
        width = 10
        height = 20

        fig,ax = plt.subplots(1)
        rect = mpatches.Rectangle((x, y), width, height)

        ax.add_patch(rect)
        self.axes.plot(rect)

Can someone help me? Any help would be greatly appreciated!