How to plot confusion matrix with gridding without color using matplotlib?

Question

I found a similar quesion on How to plot confusion matrix with string axis rather than integer in python. But the answer is not exact what I want. Because it doesn't contain gridding (e.g., the numbers are not in little squares) and there is background color to show the number which is not what I want.

import numpy as np
import matplotlib.pyplot as plt

conf_arr = [[33,2,0,0,0,0,0,0,0,1,3], 
            [3,31,0,0,0,0,0,0,0,0,0], 
            [0,4,41,0,0,0,0,0,0,0,1], 
            [0,1,0,30,0,6,0,0,0,0,1], 
            [0,0,0,0,38,10,0,0,0,0,0], 
            [0,0,0,3,1,39,0,0,0,0,4], 
            [0,2,2,0,4,1,31,0,0,0,2],
            [0,1,0,0,0,0,0,36,0,2,0], 
            [0,0,0,0,0,0,1,5,37,5,1], 
            [3,0,0,0,0,0,0,0,0,39,0], 
            [0,0,0,0,0,0,0,0,0,0,38]]

norm_conf = []
for i in conf_arr:
    a = 0
    tmp_arr = []
    a = sum(i, 0)
    for j in i:
        tmp_arr.append(float(j)/float(a))
    norm_conf.append(tmp_arr)

fig = plt.figure()
plt.clf()
ax = fig.add_subplot(111)
ax.set_aspect(1)
res = ax.imshow(np.array(norm_conf), cmap=plt.cm.jet, 
                interpolation='nearest')

width, height = conf_arr.shape

for x in xrange(width):
    for y in xrange(height):
        ax.annotate(str(conf_arr[x][y]), xy=(y, x), 
                    horizontalalignment='center',
                    verticalalignment='center')

cb = fig.colorbar(res)
alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
plt.xticks(range(width), alphabet[:width])
plt.yticks(range(height), alphabet[:height])
plt.savefig('confusion_matrix.png', format='png')

Oliver W. · Accepted Answer · 2016-05-29T15:37:18.087

By making only a few changes to that rather excellent code proposal (I upvoted it, consider doing that too), you can get the figure you're describing.

You'll get gridding by calling the hlines and vlines methods of the ax object, which will add horizontal and vertical lines respectively. When you then also remove the call to imshow, the colors are gone. Like this:

import numpy as np
import matplotlib.pyplot as plt

conf_arr = np.array([[33,2,0,0,0,0,0,0,0,1,3], 
            [3,31,0,0,0,0,0,0,0,0,0], 
            [0,4,41,0,0,0,0,0,0,0,1], 
            [0,1,0,30,0,6,0,0,0,0,1], 
            [0,0,0,0,38,10,0,0,0,0,0], 
            [0,0,0,3,1,39,0,0,0,0,4], 
            [0,2,2,0,4,1,31,0,0,0,2],
            [0,1,0,0,0,0,0,36,0,2,0], 
            [0,0,0,0,0,0,1,5,37,5,1], 
            [3,0,0,0,0,0,0,0,0,39,0], 
            [0,0,0,0,0,0,0,0,0,0,38]])
height, width = conf_arr.shape

fig = plt.figure('confusion matrix')
ax = fig.add_subplot(111, aspect='equal')
for x in range(width):
    for y in range(height):
        ax.annotate(str(conf_arr[x][y]), xy=(y, x), ha='center', va='center')

offset = .5    
ax.set_xlim(-offset, width - offset)
ax.set_ylim(-offset, height - offset)

ax.hlines(y=np.arange(height+1)- offset, xmin=-offset, xmax=width-offset)
ax.vlines(x=np.arange(width+1) - offset, ymin=-offset, ymax=height-offset)

alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
plt.xticks(range(width), alphabet[:width])
plt.yticks(range(height), alphabet[:height])
plt.savefig('confusion_matrix.png', format='png')

Remark that when you remove the call to imshow, you'll need to set the x- and y-limits explicitly, as shown above, otherwise you'll only see the lower left region (imshow updates the limits automatically depending on what you pass to it).

How to plot confusion matrix with gridding without color using matplotlib?

1 Answers1