Adjusting Matplotlib Drawing Order

Introduction

In this tutorial, we will learn about the drawing order of artists in Matplotlib and how to adjust the order using the zorder attribute. We will also explore how to change the order for individual artists and the default value of zorder for different types of artists.

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Understanding Zorder

The zorder attribute in Matplotlib is a floating point number that determines the drawing order of artists. Artists with higher zorder are drawn on top of those with lower zorder. The default value of zorder depends on the type of the artist. For example, images have a default zorder of 0, while patches have a default zorder of 1.

Changing Zorder

To change the drawing order of artists, we can set their zorder attribute explicitly using the zorder parameter when creating the artist. For example, we can move dots on top of lines in a scatter plot by setting the zorder of the dots to a value higher than the zorder of the line.

import matplotlib.pyplot as plt
import numpy as np

r = np.linspace(0.3, 1, 30)
theta = np.linspace(0, 4*np.pi, 30)
x = r * np.sin(theta)
y = r * np.cos(theta)

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(6, 3.2))

ax1.plot(x, y, 'C3', lw=3)
ax1.scatter(x, y, s=120)
ax1.set_title('Lines on top of dots')

ax2.plot(x, y, 'C3', lw=3)
ax2.scatter(x, y, s=120, zorder=2.5)  ## move dots on top of line
ax2.set_title('Dots on top of lines')

plt.tight_layout()
plt.show()

Setting Zorder for Ticks and Grid Lines

We can use the set_axisbelow() method or the axes.axisbelow parameter to set the zorder of ticks and grid lines.

ax = plt.axes()
ax.plot([1, 2, 3], [2, 4, 3])
ax.set_axisbelow(True)
ax.yaxis.grid(color='gray', linestyle='dashed')

Custom Order of Elements

We can also set the zorder of elements in a custom order. For example, we can set the zorder of a legend to be between two lines.

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 7.5, 100)
plt.rcParams['lines.linewidth'] = 5
plt.figure()
plt.plot(x, np.sin(x), label='zorder=2', zorder=2)  ## bottom
plt.plot(x, np.sin(x+0.5), label='zorder=3',  zorder=3)
plt.axhline(0, label='zorder=2.5', color='lightgrey', zorder=2.5)
plt.title('Custom order of elements')
l = plt.legend(loc='upper right')
l.set_zorder(2.5)  ## legend between blue and orange line
plt.show()

Summary

In this tutorial, we learned about the zorder attribute in Matplotlib and how to change the drawing order of artists. We also explored how to set the zorder for ticks and grid lines and create a custom order of elements. Understanding zorder is essential when creating complex visualizations with overlapping elements.