matplotlib two y axes with mixed z order

Question

I would like to plot data on two y axes such that some of the data on the second y axis is behind the first y axis graph and part of it is above. Essentially I would like to have use "global" zorder parameter. Is that possible?

Here is a minimal example:

import numpy as np
import matplotlib.pyplot as plt

# generate data
x = np.linspace(0,30,30)
y1 = np.random.random(30)+x
y2 = np.random.random(30)+x*2

# create figure
fig, ax = plt.subplots()

# y1 axis
ax.plot(x,y1,lw=5,c='#006000', zorder=2)
ax.set_ylim((0,30))
ax.set_xlim((0,30))

# y2 axis
ax2 = ax.twinx()  # instantiate a second axes that shares the same x-axis
ax2.fill_between([0, 30], [10, 10], color='pink', lw=0, zorder=1)
ax2.fill_between([0, 30], [60, 60], y2=[10, 10], color='gray', lw=0, zorder=1)
ax2.plot(x, y2,'o',ms=3,c='black', zorder=3)
ax2.set_ylim((0,60))
ax2.set_xlim((0,30))

# move y1 axis to the front
ax.set_zorder(ax2.get_zorder()+1)
ax.patch.set_visible(False)

I would like the background fill color to be in the background but the black data points should be on top of the green line. I tried to achieve this by defining the zorder parameter for these curves but apparently the zorder is only defined within one axis and not across multiple axes.

Answer 1

Here is a solution that gets what you want, however sub-ideal it may be in implementation.

import numpy as np
import matplotlib.pyplot as plt

# generate data
x = np.linspace(0,30,30)
y1 = np.random.random(30)+x
y2 = np.random.random(30)+x*2

# create figure
fig, ax1 = plt.subplots()
ax2 = ax1.twinx()
ax3 = ax2.twiny()
ax1.get_shared_x_axes().join(ax2, ax3)

# line
ax1.plot(x,y1,lw=5,c='#006000')
ax1.set_ylim((0,30))
ax1.set_xlim((0,30))

# points
ax2.plot(x, y2,'o',ms=3,c='black')
ax2.set_ylim((0,60))

# fills
ax3.set_xticklabels([])
ax3.get_xaxis().set_visible(False)
ax3.fill_between([0, 30], [10, 10], color='pink', lw=0)
ax3.fill_between([0, 30], [60, 60], y2=[10, 10], color='gray', lw=0)

# order
ax3.zorder = 1 # fills in back
ax1.zorder = 2 # then the line
ax2.zorder = 3 # then the points
ax1.patch.set_visible(False)

plt.show()

Answer 2

It seems there is a clear relationship between the two axes (in this case a factor of 2). So one could plot everything in the same axes and just scale the necessary parts by the factor. (This requires matplotlib >= 3.1)

import numpy as np
import matplotlib.pyplot as plt

# generate data
x = np.linspace(0,30,30)
y1 = np.random.random(30)+x
y2 = np.random.random(30)+x*2

# create figure
fig, ax = plt.subplots()

f = lambda x: 2*x
g = lambda x: x/2
ax2 = ax.secondary_yaxis('right', functions=(f,g))

ax.plot(x, y1,lw=5,c='#006000', zorder=2)
ax.plot(x, g(y2),'o',ms=3,c='black', zorder=3)
ax.set_ylim((0,30))
ax.set_xlim((0,30))


ax.fill_between([0, 30], [5, 5], color='pink', lw=0, zorder=1)
ax.fill_between([0, 30], [30, 30], y2=[5, 5], color='gray', lw=0, zorder=0)

plt.show()