Plot the histogram with matplotlib, returns `matplotlib` figure.

def plot(self, ax=None, **kwargs):
        """
        Plot the histogram with matplotlib, returns `matplotlib` figure.
        """
        ax, fig, plt = get_ax_fig_plt(ax)

        yy = [len(v) for v in self.values]
        ax.plot(self.binvals, yy, **kwargs)

        return fig