Matplotlib axis with two scales shared origin

Question

I need two overlay two datasets with different Y-axis scales in Matplotlib. The data contains both positive and negative values. I want the two axes to share one origin, but

Answer 1

The other answers here seem overly complicated and don't necessarily work for all the scenarios (e.g. ax1 is all negative and ax2 is all positive). There are 2 easy methods that always work:

1. Always put 0 in the middle of the graph for both y axes
2. A bit fancy and somewhat preserves the positive-to-negative ratio, see below

def align_yaxis(ax1, ax2):
    y_lims = numpy.array([ax.get_ylim() for ax in [ax1, ax2]])

    # force 0 to appear on both axes, comment if don't need
    y_lims[:, 0] = y_lims[:, 0].clip(None, 0)
    y_lims[:, 1] = y_lims[:, 1].clip(0, None)

    # normalize both axes
    y_mags = (y_lims[:,1] - y_lims[:,0]).reshape(len(y_lims),1)
    y_lims_normalized = y_lims / y_mags

    # find combined range
    y_new_lims_normalized = numpy.array([numpy.min(y_lims_normalized), numpy.max(y_lims_normalized)])

    # denormalize combined range to get new axes
    new_lim1, new_lim2 = y_new_lims_normalized * y_mags
    ax1.set_ylim(new_lim1)
    ax2.set_ylim(new_lim2)