matplotlibscaleaxis

Matplotlib axis with two scales shared origin


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 Matplotlib does not align the two scales by default.

import numpy as np
import matplotlib.pyplot as plt

fig = plt.figure()
ax1 = fig.add_subplot(111)
ax2 = ax1.twinx()

ax1.bar(range(6), (2, -2, 1, 0, 0, 0))
ax2.plot(range(6), (0, 2, 8, -2, 0, 0))
plt.show()

I suppose it is possible to perform some computation with .get_ylim() and .set_ylim() two align the two scales. Is there an easier solution?

Output from the sample above


Solution

  • use the align_yaxis() function:

    import numpy as np
    import matplotlib.pyplot as plt
    
    def align_yaxis(ax1, v1, ax2, v2):
        """adjust ax2 ylimit so that v2 in ax2 is aligned to v1 in ax1"""
        _, y1 = ax1.transData.transform((0, v1))
        _, y2 = ax2.transData.transform((0, v2))
        inv = ax2.transData.inverted()
        _, dy = inv.transform((0, 0)) - inv.transform((0, y1-y2))
        miny, maxy = ax2.get_ylim()
        ax2.set_ylim(miny+dy, maxy+dy)
    
    
    fig = plt.figure()
    ax1 = fig.add_subplot(111)
    ax2 = ax1.twinx()
    
    ax1.bar(range(6), (2, -2, 1, 0, 0, 0))
    ax2.plot(range(6), (0, 2, 8, -2, 0, 0))
    
    align_yaxis(ax1, 0, ax2, 0)
    plt.show()
    

    enter image description here