How to deal with data of class uneval error from ggplot2

Question

While trying to overlay a new line to a existing ggplot I am getting the following error    Error  ggplot2 doesn t know how to deal with data of class uneval   The first part of my code works fine   Below is a image of  recent  hourly wind generation data from a Midwestern United States electric power market     Now I want to overlay the last two days worth of observations in Red   It should be easy but I cant figure out why I am getting a error   Any assistance would be greatly appreciated   Below is a reproducable example     Read in Wind data fname  lt -  https   www midwestiso org Library Repository Market 20Reports 20130510 hwd HIST csv  df  lt - read csv fname  header TRUE  sep       skip 7  df  lt - df 1  length df MKTHOUR -5       format variables df MWh  lt - as numeric df MWh  df Datetime  lt - strptime df MKTHOUR    m  d  y  I  M  p      Create some variables df Date   lt - as Date df Datetime  df HrEnd  lt - df Datetime hour 1    Subset recent and last data last obs   lt - range df Date  2  df recent  lt - subset df  Date  in  seq last obs-30  last obs-2  by 1   df last    lt - subset df  Date  in  seq last obs-2   last obs    by 1      plot recent in Grey p  lt - ggplot df recent  aes HrEnd  MWh  group factor Date         geom line color  grey       scale y continuous labels   comma       scale x continuous breaks   seq 1 24 1       labs y  MWh        labs x  Hour Ending        labs title  Hourly Wind Generation       p    plot last two days in Red p  lt - p   geom line df last  aes HrEnd  MWh  group factor Date    color  red     p

User · Accepted Answer

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")

User · Answer

Another cause is accidentally putting the data     inside the aes      instead of outside   RIGHT  ggplot data df df var7   9-06     aes x lifetime y rep rate group mdcp color mdcp        WRONG  ggplot aes data df df var7   9-06    x lifetime y rep rate group mdcp color mdcp         In particular this can happen when you prototype your plot command with qplot    which doesn t use an explicit aes    then edit copy-and-paste it into a ggplot    qplot data      x     y            ggplot data      aes x     y             It s a pity ggplot s error message isn t Missing  data  argument  instead of this cryptic nonsense  because that s what this message often means

User · Answer

This could also occur if you refer to a variable in the data frame that doesn t exist  For example  recently I forgot to tell ddply to summarize by one of my variables that I used in geom line to specify line color  Then  ggplot didn t know where to find the variable I hadn t created in the summary table  and I got this error

[r] How to deal with "data of class uneval" error from ggplot2?

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