fix for empty label warning

fixes the

No artists with labels found to put in legend.  Note that artists whose label start with an underscore are ignored when legend() is called with no argument.

the legend look like so if no flags are plotted:

approved this merge request

merged

mentioned in commit a9cd4648

mentioned in commit 63b55c6d

mentioned in commit 684dc8a0

mentioned in commit 8f7a90e4