Creating Contour Plots with Python Matplotlib

Introduction

This lab is a step-by-step tutorial on how to create contour plots using Python Matplotlib. Contour plots are useful for visualizing three-dimensional data in two dimensions. In this tutorial, we will be illustrating simple contour plotting, contours on an image with a colorbar for the contours, and labeled contours.

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Skills Graph

Import Required Libraries

Before we can begin creating our contour plot, we need to import the necessary libraries. We will be using numpy and matplotlib for this tutorial.

import matplotlib.pyplot as plt
import numpy as np

Create Data

We need to create the data we will be using to create the contour plot. In this example, we will be creating two 2D Gaussian functions.

delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = (Z1 - Z2) * 2

Create a Simple Contour Plot with Labels

Now that we have our data, we can create a simple contour plot with labels using default colors.

fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z)
ax.clabel(CS, inline=True, fontsize=10)
ax.set_title('Simplest default with labels')

Place Contour Labels Manually

We can also place contour labels manually by providing a list of positions (in data coordinate).

fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z)
manual_locations = [
    (-1, -1.4), (-0.62, -0.7), (-2, 0.5), (1.7, 1.2), (2.0, 1.4), (2.4, 1.7)]
ax.clabel(CS, inline=True, fontsize=10, manual=manual_locations)
ax.set_title('labels at selected locations')

Set Contour Colors

We can force all the contours to be the same color or set negative contours to be solid instead of dashed.

fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z, 6, colors='k')  ## Negative contours default to dashed.
ax.clabel(CS, fontsize=9, inline=True)
ax.set_title('Single color - negative contours dashed')

plt.rcParams['contour.negative_linestyle'] = 'solid'
fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z, 6, colors='k')  ## Negative contours default to dashed.
ax.clabel(CS, fontsize=9, inline=True)
ax.set_title('Single color - negative contours solid')

Manually Specify Contour Colors

We can also manually specify the colors of the contour.

fig, ax = plt.subplots()
CS = ax.contour(X, Y, Z, 6,
                linewidths=np.arange(.5, 4, .5),
                colors=('r', 'green', 'blue', (1, 1, 0), '#afeeee', '0.5'),
                )
ax.clabel(CS, fontsize=9, inline=True)
ax.set_title('Crazy lines')

Use a Colormap to Specify Contour Colors

We can use a colormap to specify the colors for the contour lines.

fig, ax = plt.subplots()
im = ax.imshow(Z, interpolation='bilinear', origin='lower',
               cmap=cm.gray, extent=(-3, 3, -2, 2))
levels = np.arange(-1.2, 1.6, 0.2)
CS = ax.contour(Z, levels, origin='lower', cmap='flag', extend='both',
                linewidths=2, extent=(-3, 3, -2, 2))

## Thicken the zero contour.
CS.collections[6].set_linewidth(4)

ax.clabel(CS, levels[1::2],  ## label every second level
          inline=True, fmt='%1.1f', fontsize=14)

## make a colorbar for the contour lines
CB = fig.colorbar(CS, shrink=0.8)

ax.set_title('Lines with colorbar')

## We can still add a colorbar for the image, too.
CBI = fig.colorbar(im, orientation='horizontal', shrink=0.8)

## This makes the original colorbar look a bit out of place,
## so let's improve its position.

l, b, w, h = ax.get_position().bounds
ll, bb, ww, hh = CB.ax.get_position().bounds
CB.ax.set_position([ll, b + 0.1*h, ww, h*0.8])

Summary

In this lab, we have learned how to create contour plots using Python Matplotlib. We have covered creating a simple contour plot with labels, placing contour labels manually, setting contour colors, manually specifying contour colors, and using a colormap to specify contour colors.