Just had a thought and I am curious about what others think.
Now that we have agreed that calling a plotting function with empty data should always be considered valid, should it also automatically advance the colorcycle? I think it should.
Here is a use-case: consider a user who is plotting temperature in three regions over time in one subplot, and surface pressure over time for those same three regions. Let’s say the thermometer in second region started reporting only NaNs. If empty data did not advance the color cycle, then the line for the temperature plot of the third region will be same as the line for the pressure plot for the second region, leading to mis-leading interpretation that the thermometer in the third region was the one that broke.
This is a really simple example, but I can see this being harder to ensure for more complicated plots the depend on the automatic color cycling.