ValueError exception when plotting a legend on an empty scatter graph with marker size = data length

[7hibault]

Issue raised on Stack Overflow by burak1000: Legends for scatter plots with NANs in matplotlib

When a scatter plot is created with (x=[NaN], y=[NaN]), a call to legend will raise a ValueError exception. The error appears when the two conditions are met while calling scatter:

1. The length of s= (marker size) is equal to the data length
2. No points are plotted, i. e. for each index, x[index] or y[index] is NAN or INFINITY
   For example, this will also raise the exception:
   scatter([NAN, 2], [3, NAN], marker="o", s=[3, 4])

burak1000 provided more information about the issue on SO!

[tacaswell]

Oddly, this works correctly with empty lists, but not with nans floating around, but I suspect that this is due to some nastyness in handling the normalization of the input to scatter.

Have a PR on the way.