Colouring the area between two step lines with crossovers

Question

I am trying to fill colours between two-step line plots. I have tried to do the same using fill_between function with step and interpolate parameters. However, I am not getting the output as expected. I am filling the region between two lines after comparing their values. Below is the code. Any help will be appreciated.

fig = plt.figure()
fig.tight_layout()
plt.subplot(2, 2, 1)
p1 = plt.step(df2['datetime'], df2['T1'], color='b', linewidth=3, where = 'post', label ='P1')
p2 = plt.step(df2['datetime'], df2['T3'], color='m', linewidth=3, where = 'post', label ='P2')
p3 = plt.fill_between(df2['datetime'], df2['T1'],df2['T3'], where = df2['T1'] < 
df2['T3'],facecolor="blue", color='blue', alpha=0.25, step = 'post',interpolate = True ,label ='A1')
p4 = plt.fill_between(df2['datetime'], df2['T1'],df2['T3'], where = df2['T1'] > 
df2['T3'],facecolor="red", color='red', alpha=0.25, step = 'post',interpolate = True, label ='A2')
plt.ylabel("T1", fontsize=12, color='black')
plt.xlabel("Hour", fontsize=12, color='black')
plt.grid(True)
plt.legend(loc='best',fontsize = 10)
plt.xticks(rotation = 90)

plt.subplot(2, 2, 2)
p1 = plt.step(df2['datetime'], df2[‘T2’], color='k', linewidth=3, where = 'post', label ='P1')
p2 = plt.step(df2['datetime'], df2['T3'], color='m', linewidth=3, where = 'post', label ='P2')
p3 = plt.fill_between(df2['datetime'], df2[‘T2’],df2['T3'], where = df2[‘T2’] < 
df2['T3'],facecolor="blue", color='blue', alpha=0.25, step = 'post',label ='A1')
p4 = plt.fill_between(df2['datetime'], df2[‘T2’],df2['T3'], where = df2[‘T2’] > 
df2['T3'],facecolor="red", color='red', alpha=0.25, step = 'post',label ='A2')
plt.ylabel("T2", fontsize=12, color='black')
plt.xlabel("Hour", fontsize=12, color='black')
plt.grid(True)
plt.legend(loc='best',fontsize = 10)
plt.xticks(rotation = 90)

I am also attaching my output for reference.

Left plot is with step = post and interpolate = True Right plot is without interpolate. As you can see filling in not working as expected near cross overs.

Answer 1

Apparently, fill_between between step plots and using a where parameter doesn't fill as expected.

A workaround is to mimic the step function via np.repeat :

import matplotlib.pyplot as plt
import numpy as np

x = np.arange(20)
y1 = np.random.rand(20)
y2 = np.random.rand(20)
xx = np.repeat(x, 2)[1:]
yy1 = np.repeat(y1, 2)[:-1]
yy2 = np.repeat(y2, 2)[:-1]

fig, (ax1, ax2, ax3) = plt.subplots(ncols=3, figsize=(18, 4))

for ax in (ax1, ax2, ax3):
    ax.step(x, y1, color='r', lw=3, where='post')
    ax.step(x, y2, color='b', lw=3, where='post')

ax1.fill_between(x, y1, y2, color='b', alpha=0.3, step='post', where=y1 < y2)
ax1.fill_between(x, y1, y2, color='r', alpha=0.3, step='post', where=y1 > y2)
ax1.set_title('fill_between with step and where')

ax2.fill_between(x, y1, y2, color='b', alpha=0.3, step='post', where=y1 < y2, interpolate=True)
ax2.fill_between(x, y1, y2, color='r', alpha=0.3, step='post', where=y1 > y2, interpolate=True)
ax2.set_title('setting interpolate=True')

ax3.fill_between(xx, yy1, yy2, color='b', alpha=0.3, where=yy1 < yy2)
ax3.fill_between(xx, yy1, yy2, color='r', alpha=0.3, where=yy1 > yy2)
ax3.set_title('mimicking step')

plt.tight_layout()
plt.show()