How to fill the area under a step curve using pyplot? (Matplotlib)

To fill the area under a step curve using pyplot, you can use the fill_between() method with the step parameter. This creates filled regions beneath step plots, which are useful for displaying discrete data or histogram-like visualizations.

Basic Step Curve with Fill

Here's how to create a simple step curve with filled area ?

import matplotlib.pyplot as plt
import numpy as np

# Set figure size
plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

# Create data points
x = np.linspace(-10, 10, 100)
y1 = np.sin(x)
y2 = np.cos(x)

# Fill area under step curves
plt.fill_between(x, y1, step="pre", alpha=0.4, label='sin(x)')
plt.fill_between(x, y2, step="pre", alpha=0.4, label='cos(x)')

# Plot step lines
plt.plot(x, y1, drawstyle="steps", color='blue')
plt.plot(x, y2, drawstyle="steps", color='orange')

plt.legend()
plt.grid(True, alpha=0.3)
plt.show()

Step Parameter Options

The step parameter in fill_between() controls how the fill connects points ?

import matplotlib.pyplot as plt
import numpy as np

x = np.array([1, 2, 3, 4, 5])
y = np.array([2, 4, 1, 5, 3])

fig, axes = plt.subplots(1, 3, figsize=(12, 4))

# step="pre" - step occurs before the point
axes[0].fill_between(x, y, step="pre", alpha=0.5)
axes[0].plot(x, y, 'o-', drawstyle="steps-pre")
axes[0].set_title('step="pre"')
axes[0].grid(True, alpha=0.3)

# step="mid" - step occurs at the midpoint
axes[1].fill_between(x, y, step="mid", alpha=0.5)
axes[1].plot(x, y, 'o-', drawstyle="steps-mid")
axes[1].set_title('step="mid"')
axes[1].grid(True, alpha=0.3)

# step="post" - step occurs after the point
axes[2].fill_between(x, y, step="post", alpha=0.5)
axes[2].plot(x, y, 'o-', drawstyle="steps-post")
axes[2].set_title('step="post"')
axes[2].grid(True, alpha=0.3)

plt.tight_layout()
plt.show()

Filling Between Two Step Curves

You can fill the area between two step curves by specifying both y1 and y2 parameters ?

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 10, 20)
y1 = np.sin(x)
y2 = np.cos(x)

plt.figure(figsize=(10, 6))

# Fill area between two step curves
plt.fill_between(x, y1, y2, step="pre", alpha=0.3, color='green', 
                 label='Area between sin(x) and cos(x)')

# Plot the step curves
plt.plot(x, y1, drawstyle="steps-pre", color='blue', label='sin(x)')
plt.plot(x, y2, drawstyle="steps-pre", color='red', label='cos(x)')

plt.xlabel('X values')
plt.ylabel('Y values')
plt.title('Filled Area Between Two Step Curves')
plt.legend()
plt.grid(True, alpha=0.3)
plt.show()

Key Parameters

Conclusion

Use fill_between() with step="pre" to fill areas under step curves. Combine with drawstyle="steps" in plot() for complete step visualizations. The alpha parameter controls transparency for better visual overlap.