Plot a matrix in Python with custom colors

Question

I'm trying to plot a matrix using pytlot in python.

import matplotlib.pyplot as plt
m = [
    [1, 0, 2, 0, 0],
    [1, 1, 1, 2, 0],
    [0, 4, 1, 0, 0],
    [0, 4, 4, 1, 2],
    [1, 3, 0, 0, 1],
]

plt.imshow(m)

plt.show()

This look like what I need, but the problem is that I need to be able to select the color for a position.

For example:

the position 1x2 has to be red.

anyway to archive this with Python and Pyplot?

Use RGB channels.. eg. your array has the shape (rows, cols, channel) and set each channel to the desired red, green, and blue value. So m[1][2[0] = 255 — Jonathan R
– Jonathan R, Commented Apr 10, 2018 at 20:04
If 1x2 is red, do you want everything that has a value of 1 to be red as well? — user3483203
– user3483203, Commented Apr 10, 2018 at 20:13

user3483203 · Accepted Answer · 2018-04-10 20:36:37Z

2

You can map a certain range to a certain colormap using the vmin and vmax arguments, and then assign those colors to particular values in your graph. Here is a simple example, using the brg cmap.

I used 1-10 as a scale, so 1 will be Blue, 5 will be Red, and 10 will be Green, etc.

import matplotlib.pyplot as plt

m = [
    [1, 0, 2, 0, 0],
    [1, 1, 1, 2, 0],
    [0, 4, 1, 0, 0],
    [0, 4, 4, 1, 2],
    [1, 3, 0, 0, 1]
]

dct = {1: 5., 0: 1., 2: 1., 3: 1., 4: 1.}
n = [[dct[i] for i in j] for j in m]
print(n)

plt.imshow(n, cmap='brg', vmin=1, vmax=10)
plt.show()

Output (All 1's in your matrix are now red, all other values set to blue):

answered Apr 10, 2018 at 20:36

user3483203

51.3k10 gold badges72 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

sciroccorics · Accepted Answer · 2018-04-10 20:51:58Z

2

I guess that matplotlib.colors.ListedColormap is what you need here:

import matplotlib
import matplotlib.pyplot as plt

colors = 'lime red blue magenta yellow'.split()
cmap = matplotlib.colors.ListedColormap(colors, name='colors', N=None)

m = [
    [1, 0, 2, 0, 0],
    [1, 1, 1, 2, 0],
    [0, 4, 1, 0, 0],
    [0, 4, 4, 1, 2],
    [1, 3, 0, 0, 1],
]

plt.imshow(m, cmap=cmap)
plt.show()

You may also use (R,G,B) tuples (instead of predefined color strings) in your list if you need more control

edited Apr 10, 2018 at 20:51

answered Apr 10, 2018 at 20:38

sciroccorics

2,4271 gold badge11 silver badges23 bronze badges

4 Comments

user3483203 Over a year ago

You might want to specifify how colors are picked for values, since this has position (1,2) as green. It seems like just a simple re-order of your listed color map.

sciroccorics Over a year ago

@chrisz: The colors are simply mapped in order : 0 -> red, 1 -> lime, etc. So you need to provide one color for each different possible value in the range (five colors here). This is easy for matrices in an integer range domain as is the case here. If your domain is real values, other colormaps schemes are preferable

user3483203 Over a year ago

I'm saying that it is assigning 0-> red when he wants 1's as red. But I like this answer, if you have a fixed domain makes it very simple.

sciroccorics Over a year ago

@chrisz: I edited the code (by switching 'red' and 'lime' so as 0 -> lime and 1 -> red. I don't know the desired colors for the other values...

Collectives™ on Stack Overflow

Plot a matrix in Python with custom colors

2 Answers 2

Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related