There is also a nice function get_legend_handles_labels() you can call on the last axis (if you iterate over them) that would collect everything you need from label= arguments:
handles, labels = ax.get_legend_handles_labels()fig.legend(handles, labels, loc='upper center')If the pyplot interface is being used instead of the Axes interface, use:
handles, labels = plt.gca().get_legend_handles_labels()To remove legends from subplots, see Remove the legend on a matplotlib figure.
To merge txinx legends, see Secondary axis with twinx(): how to add to legend.