Bug report

Bug summary

In 3.2, scatter is color-mapping 4 points when the c input is 2D with shape (1, 4). This is a change from 3.1.

The docs for scatter say

Note that c should not be a single numeric RGB or RGBA sequence
because that is indistinguishable from an array of values to be
colormapped. If you want to specify the same RGB or RGBA value for
all points, use a 2-D array with a single row. Otherwise, value-
matching will have precedence in case of a size matching with x
and y.

However, this is no longer true.

I suspect #17245 may be related (only based on milestone and description, I haven't debugged the code).

Code for reproduction

import matplotlib.pyplot as plt

x = y = 1, 2, 3, 4
c = [(.2, .5, .8, 1)]

f, ax = plt.subplots()
ax.scatter(x, y, c=c, s=100)  # Increase s for visibility

Actual outcome

On 3.2.1

Expected outcome

On 3.1.2:

Matplotlib version

• Operating system: macos
• Matplotlib version: 3.2.1
• Matplotlib backend (print(matplotlib.get_backend())): pylab inline
• Python version: na
• Jupyter version (if applicable): na
• Other libraries: na