center figure.suptitle with axes.set_title

For a figure with just one subplot, I want to have a larger main title
(using figure.suptitle) and a smaller subtitle (using axes.set_title).
However, using horizontalalignment = 'center' on both the suptitle
and title doesn't center the two relative to each other, because my
subplot varies in width (I change it depending on the y axis labels'

How can I center the suptitle appropriately over the subplot?