35

Say I am plotting a set of points with an image as a background. I've used the Lena image in the example:

import numpy as np
import matplotlib.pyplot as plt
from scipy.misc import imread

np.random.seed(0)
x = np.random.uniform(0.0,10.0,15)
y = np.random.uniform(0.0,10.0,15)
img = imread("lena.jpg")
plt.scatter(x,y,zorder=1)
plt.imshow(img,zorder=0)
plt.show()

This gives meenter image description here .

My question is: How can I specify the corner coordinates of the image in the plot? Let's say I'd like the bottom-left corner to be at x, y = 0.5, 1.0 and the top-right corner to be at x, y = 8.0, 7.0.

Trenton McKinney
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YXD
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2 Answers2

47

Use the extent keyword of imshow. The order of the argument is [left, right, bottom, top]

import numpy as np
import matplotlib.pyplot as plt

np.random.seed(0)
x = np.random.uniform(0.0,10.0,15)
y = np.random.uniform(0.0,10.0,15)

datafile = 'lena.jpg'
img = plt.imread(datafile)
plt.scatter(x,y,zorder=1)
plt.imshow(img, zorder=0, extent=[0.5, 8.0, 1.0, 7.0])
plt.show()

enter image description here

  • For cases where it's desired to have an image in a small area of the scatter plot, change the order of the plots (.imshow then .scatter) and change the extent values.
plt.imshow(img, zorder=0, extent=[3.0, 5.0, 3.0, 4.50])
plt.scatter(x, y, zorder=1)
plt.show()

enter image description here

Trenton McKinney
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David Zwicker
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    Works great. The only thing I had to add is to set `plt.xlim` and `plt.ylim` otherwise the viewport/axes are fitted tightly to the background image. – YXD Mar 01 '13 at 14:41
  • Nice solution! Thank you YXD! Just wanted to add in case someone encountered the same: if you do plt.scatter(), then plt.imshow(), the figure may be covered by the image, with sized defined in "extent" kwarg. If this is the case you had, try: do plt.imshow() first, then plt.scatter(). Then you will have a figure, with size determined by your data (x,y), and images displayed on the graph. – Ruby Nov 21 '16 at 00:32
11

You must use the extent keyword parameter:

imshow(img, zorder=0, extent=[left, right, bottom, top])

The elements of extent should be specified in data units so that the image can match the data. This can be used, for example, to overlay a geographical path (coordinate array) over a geo-referenced map image.

heltonbiker
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