Label a subplot with shared axis

How can you label the bottom subplot if using shared axes? For example, if I have a plot with 2 rows and 1 column and a shared x-axis, if I try to assign the title to xaxis1, it labels the top subplot instead of the bottom one. However, there is only an xaxis1 since it is shared…what is the proper way to do so using the python API?

Annotations are best for labeling subplots currently.