I think I ran into a couple of bugs of the Digital Waveform Graph, demonstrated by the attached VI (LV2015 linux). The probable bugs are:
plot names set programmatically are not immediately effected; the correct names are updated only after some refresh or plot rescaling action;
Of a minour level of nuisance, the visibility of Plot Legends in either Standard or Tree view is inconsistently toggled, in edit mode, by left click action. The attached VI is saved with a Standard legend visible which is not supposed to be visible according to Visible Items/Plot Legend. To hide it, the detour I've found is: Visible Items/tag Plot Legend (the tree becomes visible); Advanced/Change Plot Legend to standard view; Visible Items/untoggle Plot Legend;
Thank you for reporting this Enrico_Segre. I was able to replicate the bug using the VI you attached so I went ahead and filed a CAR (570134). You can look for a fix in further patch notes under that CAR number.
As for the plot legends, I am going to do a little more research on if that is a bug or intended. I will update you with what I find.