matplotlib.scatter() not working with Numpy on Python 3.6

Use np.reshape:

import numpy as np

t1 = np.array([[1,2,3,4,5,6] , [7,8,9,10,11,12]])
t1_single = np.reshape(t1, -1)
print(t1.shape)
print(t1_single.shape)

Output:
(2, 6)
(12,)

plt.scatter(X[:, 0], X[:, 1], s=40, c=T, cmap=plt.cm.Spectral)

In this function, c requires 1-D array, As mentioned in above answer, use T.ravel or T.reshape(400,)

c requires a single dimensional array.

T.ravel() should do the trick.

You can also use c=np.squeeze(T).

I think the problem here is actually part of a bigger python/numpy problem - which is it's inability to infer the correct usage of 1D arrays. This wastes ton of times coding and debugging.