Hello, I've run into a strange behaviour of matplotlib while trying to figure out why my data was displayed incorrectly. I'm note quite sure if this is a bug or expected behaviour, but I feel it's kind of counter-intuitive, so I'm posting here.
The attached python script does a scatter plot of some data. I'm using the first column as the x coordinates and the second as y. Looking at the matrix (x == y), I'd expect the three data point to be on a diagonal line. Now there seem to be a difference on how numpy handles A[:,0] depending on if A is a np.array or np.matrix. In the case of an array, a 1D array is returned, in the case of a matrix, a 2D Nx1 matrix is returned. Using this matrix seems to confuse matplotlib. Using np.ravel or np.flatten on the slices fix that problem. Is there an explanation for this behaviour or should I fill a bug ? Best regards, Julien Rebetez
<<attachment: matrix_plot.png>>
import pylab as pl
import numpy as np
A = np.array([[1, 1],
[2, 2],
[3, 3]])
B = np.matrix(A)
# As far as I understand, the problem seems to be caused by the fact that one
# is a 1D array and the other a 2D array (with only one column)
print A[:,1].shape
print B[:,1].shape
pl.figure()
pl.scatter(A[:,0], A[:,1], c='r')
pl.scatter(B[:,0], B[:,1], c='b')
pl.show()
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