I am embedding a ZX diagram into a web app (using shiny), and calling pyzx.draw(g) results in an error 'NoneType' object has no attribute 'rowspan'.

The cause is due to the way that PyZX is creating the figure in these lines, which does not set matplotlib.gridspec.SubplotSpec properly.

    fig1 = plt.figure(figsize=figsize)
    ax = fig1.add_axes([0, 0, 1, 1], frameon=False)

Some matplotlib environments ignore this silently, but in others this causes an error. For example, this fails also in the latest version of pandas (see this issue and this Stack Overflow answer).

This can be fixed by calling subplots instead:

    fig1, ax = plt.subplots(figsize=figsize)
    ax.set_frame_on(False)

But I think this might result in a slightly different appearance in some environments, as the axes are computed rather than explicitly set. Were the original values [0, 0, 1, 1] explicitly chosen to force a particular appearance? Otherwise, it seems it's likely more robust to let them be computed based on the environment.