Efficient Line Plotting with Matplotlib

Introduction

In this tutorial, we will learn how to use the LineCollection function in Matplotlib to efficiently draw multiple lines at once. We will see how to plot multiple lines with different colors and styles, and how to use a masked array to mask some values. We will also learn how to use the ScalarMappable.set_array function to map an array of values to colors.

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Skills Graph

Import Libraries

Before we start, we need to import the necessary libraries. We will use matplotlib.pyplot and numpy.

import matplotlib.pyplot as plt
import numpy as np

Create Data

Next, we need to create the data that we will use to plot the lines. We will use numpy to create a 2D array of x and y values.

x = np.arange(100)
ys = x[:50, np.newaxis] + x[np.newaxis, :]

Create Line Collection

Now, we can create a LineCollection object with the LineCollection function. We can set the linewidths, colors, and linestyle parameters to customize the appearance of the lines.

colors = plt.rcParams['axes.prop_cycle'].by_key()['color']

segs = np.zeros((50, 100, 2))
segs[:, :, 1] = ys
segs[:, :, 0] = x

segs = np.ma.masked_where((segs > 50) & (segs < 60), segs)

line_segments = LineCollection(segs, linewidths=(0.5, 1, 1.5, 2),
                               colors=colors, linestyle='solid')

Create Plot

We can now create a plot using matplotlib and add the LineCollection object to the plot using the add_collection method of the Axes object.

fig, ax = plt.subplots()
ax.set_xlim(x.min(), x.max())
ax.set_ylim(ys.min(), ys.max())

ax.add_collection(line_segments)
ax.set_title('Line collection with masked arrays')
plt.show()

Map Colors to Values

We can also map an array of values to colors using the ScalarMappable.set_array function. We will create a new set of data and a new LineCollection object with the array parameter set to the x values. We can then use the colorbar method of the Figure object to add a colorbar to the plot.

N = 50
x = np.arange(N)
ys = [x + i for i in x]
segs = [np.column_stack([x, y]) for y in ys]

fig, ax = plt.subplots()
ax.set_xlim(np.min(x), np.max(x))
ax.set_ylim(np.min(ys), np.max(ys))

line_segments = LineCollection(segs, array=x,
                               linewidths=(0.5, 1, 1.5, 2),
                               linestyles='solid')
ax.add_collection(line_segments)
axcb = fig.colorbar(line_segments)
axcb.set_label('Line Number')
ax.set_title('Line Collection with mapped colors')
plt.sci(line_segments)
plt.show()

Summary

In this tutorial, we learned how to use the LineCollection function in Matplotlib to efficiently draw multiple lines at once. We saw how to plot multiple lines with different colors and styles, and how to use a masked array to mask some values. We also learned how to use the ScalarMappable.set_array function to map an array of values to colors.