Area fill between two lines in Matplotlib

Once you know how to plot several lines with Matplotlib it's quite straightforward to add an area fill between them with the fill_between() function.

This posts shows how to add an area fill between two lines with the color depending on which line has a larger value.

Libraries & Dataset

Let's get started by creating some fake data and load a few libraries:

import matplotlib.pyplot as plt
import pandas as pd

data = {
   'time':range(12),
   'income':[5, 9, 6, 6, 10, 7, 6, 4, 4, 5, 6, 4],
   'expenses':[6, 6, 8, 3, 6, 9, 7, 8, 6, 6, 4, 8]
}
df = pd.DataFrame(data)
df.head()

	time	income	expenses
0	0	5	6
1	1	9	6
2	2	6	8
3	3	6	3
4	4	10	6

Plot

The code mainly relies on the fill_between(), that will fill the space given points.

# Initialize figure and axis
fig, ax = plt.subplots(figsize=(8, 8))

# Plot lines
ax.plot(df['time'], df['income'], color="green")
ax.plot(df['time'], df['expenses'], color="red")

# Fill area when income > expenses with green
ax.fill_between(
    df['time'], df['income'], df['expenses'], where=(df['income'] > df['expenses']), 
    interpolate=True, color="green", alpha=0.25, 
    label="Positive"
)

# Fill area when income <= df['expenses'] with red
ax.fill_between(
    df['time'], df['income'], df['expenses'], where=(df['income'] <= df['expenses']), 
    interpolate=True, color="red", alpha=0.25,
    label="Negative"
)

ax.legend()
plt.show()

Change interpolate value

Setting interpolate to True calculates the intersection point between the two lines and extends the filled region up to this point, which results in the better looking filled area.

Let's see how it look when it's False:

# Initialize figure and axis
fig, ax = plt.subplots(figsize=(8, 8))

# Plot lines
ax.plot(df['time'], df['income'], color="green")
ax.plot(df['time'], df['expenses'], color="red")

# Fill area when income > expenses with green
ax.fill_between(
    df['time'], df['income'], df['expenses'], where=(df['income'] > df['expenses']), 
    interpolate=False, color="green", alpha=0.25, 
    label="Positive"
)

# Fill area when income <= df['expenses'] with red
ax.fill_between(
    df['time'], df['income'], df['expenses'], where=(df['income'] <= df['expenses']), 
    interpolate=False, color="red", alpha=0.25,
    label="Negative"
)

ax.legend()
plt.show()

Going further

Contact & Edit

👋 This document is a work by Yan Holtz. You can contribute on github, send me a feedback on twitter or subscribe to the newsletter to know when new examples are published! 🔥