R ggplot2: Line overlayed on Bar Graph (from separate data frames)

Question

I have a bar graph coming from one set of monthly data and I want to overlay on it data from another set of monthly data in the form of a line. Here is a simplified example (in my data the second data set is not a simple manipulation of the first):

library(reshape2)
library(ggplot2)

test<-abs(rnorm(12)*1000)
test<-rbind(test, test+500)
colnames(test)<-month.abb[seq(1:12)]
rownames(test)<-c("first", "second")
otherTest<-apply(test, 2, mean)
test<-melt(test)
otherTest<-as.data.frame(otherTest)

p<-ggplot(test, aes(x=Var2, y=value,  fill=Var1, order=-as.numeric(Var2))) + geom_bar(stat="identity")+
    theme_bw() + theme(panel.border = element_blank(), panel.grid.major = element_blank(), 
    panel.grid.minor = element_blank(), axis.line = element_line(colour = "black")) +
    ggtitle("Test Graph") +
    scale_fill_manual(values = c(rgb(1,1,1), rgb(.9,0,0))) + 
    guides(fill=FALSE) +
    theme(axis.text.x = element_text(angle = 45, hjust = 1))

works great to get the bar graph:

but I have tried multiple iterations to get the line on there and can't figure it out (like this):

p + geom_line(data=otherTest,size=1, color=rgb(0,.5,0)

Also, if anybody knows how I can make the bars in front of each other so that all you see is a red bar of height 500, I would appreciate any suggestions. I know I can just take the difference between the two lines of the matrix and keep it as a stacked bar but I thought there might be an easy way to put both bars on the x-axis, white in front of red. Thanks!

score 1 · Accepted Answer · edited May 23 '17 at 11:53

You have a few problems to deal with here.

Directly answering your question, if you don't provide a mapping via aes(...) in a geom call (like your geom_line...), then the mapping will come from ggplot(). Your ggplot() specifies x=Var2, y=value, fill=Var1.... All of these variable names must exist in your data frame otherTest for this to work, and they don't right now.

So, you either need to ensure that these variable names exist in otherTest, or specify mapping separately in geom_line. You might want to read up about how these layering options work. E.g., here's a post of mine that goes into some detail.

If you go for the first option, some other problems to think about:

is Var2 a factor with the same levels in both data frames? It probably should be.
to use geom_line as you are, you might need to add group = 1. See here.

Some others too, but here's a brief example of what you might do:

library(reshape2)
library(ggplot2)

test <- abs(rnorm(12)*1000)
test <- rbind(test, test+500)
colnames(test) <- month.abb[seq(1:12)]
rownames(test) <- c("first", "second")
otherTest <- apply(test, 2, mean)
test <- melt(test)

otherTest <- data.frame(
  Var2 = names(otherTest),
  value = otherTest
)

otherTest$Var2 = factor(otherTest$Var2, levels = levels(test$Var2))

ggplot(test, aes(x = Var2, y = value, group = 1)) +
  geom_bar(aes(fill = Var1), stat="identity") +
  geom_line(data = otherTest)

That worked, thanks! why do you need the line that factors Var2? I took it out and it still seems to work fine. Also, any thoughts on how to get the bars one in front of the other instead of stacked? — user3390169, Feb 19 '17 at 23:36
I use factors to be certain of the order. Using strings will create the order for you (which I prefer to avoid). Re bars in front of each, you might try adding `position="identity"` — Simon Jackson, Feb 20 '17 at 00:42

R ggplot2: Line overlayed on Bar Graph (from separate data frames)

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