Matplotlib Scatter - ValueError: RGBA sequence should have length 3 or 4

Your y array looks like

 ['3']
 ['9']
 ['0']
 ['5']
 ['5']
 ['Triangle']
 ['7']
 ['9']
 ['0']
 ['0']
...

while in reality it should look like

[3,9,0,5,5,5,7,9,0,0, ...]

I also ran into this error. In my case, the issue was that I was plot-ing when I meant to scatter. Changing

plt.plot(x1, x2, c=target)

to

plt.scatter(x1, x2, c=target)

fixed it.