How to deal with “data of class uneval” error from ggplot2?

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when you add a new data set to a geom you need to use the data= argument. Or put the arguments in the proper order mapping=..., data=.... Take a look at the arguments for ?geom_line.

Thus:

p + geom_line(data=df.last, aes(HrEnd, MWh, group=factor(Date)), color="red") 

Or:

p + geom_line(aes(HrEnd, MWh, group=factor(Date)), df.last, color="red") 

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