How to deal with the "data class uneval" error from ggplot2?

Question

When I try to overlay a new line on an existing ggplot, I get the following error:

Error: ggplot2 doesn't know how to deal with data of class uneval 

The first part of my code is working fine. Below is an image of the “recent” hourly data on wind generation from the electricity market in the Midwestern United States.

Now I want to overlay the last two days of observation in Red. This should be easy, but I can’t understand why I am getting an error.

Any help would be greatly appreciated.

The following is a reproducible example:

 # Read in Wind data fname <- "https://www.midwestiso.org/Library/Repository/Market%20Reports/20130510_hwd_HIST.csv" df <- read.csv(fname, header=TRUE, sep="," , skip=7) df <- df[1:(length(df$MKTHOUR)-5),] # format variables df$MWh <- as.numeric(df$MWh) df$Datetime <- strptime(df$MKTHOUR, "%m/%d/%y %I:%M %p") # Create some variables df$Date <- as.Date(df$Datetime) df$HrEnd <- df$Datetime$hour+1 # Subset recent and last data last.obs <- range(df$Date)[2] df.recent <- subset(df, Date %in% seq(last.obs-30, last.obs-2, by=1)) df.last <- subset(df, Date %in% seq(last.obs-2, last.obs, by=1)) # plot recent in Grey p <- 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 <- p + geom_line(df.last, aes(HrEnd, MWh, group=factor(Date)), color="red") p 

Answer 1

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

In this way:

 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") 

Answer 2

Another reason is the random placement of data=... inside aes(...) instead of the outer one:

 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 the plot command with qplot() , which does not use explicit aes() , and then edits / copies and ggplot() it into ggplot()

 qplot(data=..., x=...,y=..., ...) ggplot(data=..., aes(x=...,y=...,...)) 

It is a pity that the ggplot error message did not miss the "data" argument! instead of this cryptic stupidity, because it often means this message.

Answer 3

This can also happen if you reference a variable in data.frame that does not exist. For example, I recently forgot to say ddply to summarize one of my variables, which I used in geom_line to indicate the color of the line. Then ggplot did not know where to find the variable that I did not create in the pivot table, and I got this error.