Question

For my convenience, I want to write a function that adds a line plot and a scatter plot to an already existing plot. I defined:

addlinetoplot <- function(dataset, varx, vary)
     { 
       p <- geom_line(data=dataset, aes_string(x=varx, y=vary)) + 
            geom_point(data=dataset, aes_string(x=varx, y=vary))

       p
     }

The above function does not work. It works if I remove the + sign and the geom_point() part. The above does not work because for some reason one cannot add those two geoms. The following example illustrates the problem.

Then, I tried:

df1 <- data.frame(c1 = c(1:10), c2 = c(1:10))
c1 <- c(1:10)
csq <- c1^2
df2 <- data.frame(c1 = c(1:10), csq)
pltbase <- ggplot() + geom_line(df1, aes(x="c1", y="c2")) 

# This does not work.
pltbase + addlinetoplot(dataset=df2, varx = "c1", vary = "csq")

I figured the problem is with the statement:

addthistotheplot <- geom_line(data=df2, aes_string(x="c1", y="csq")) + 
                    geom_point(data=df2, aes_string(x="c1", y="csq"))

Trying to define the above statement throws an error: non-numeric argument to binary operator.

How can I define the addlinetoplot() function so that I can make:

pltbase + addlinetoplot(dataset=df2, varx = "c1", vary = "csq")

work. One way is to separate out the geom_line and geom_point and have different function definition for each. Is there a way to do it in the same function?

Thanks!

Answer 1

I can’t find the reference right now, but there’s an answer on SO somewhere that explains you can add to a ggplot plot by passing new arguments as a list. Accordingly, your function becomes:

addlinetoplot <- function(dataset, varx, vary) { 
  list(
    geom_line(data=dataset, aes_string(x=varx, y=vary)), 
    geom_point(data=dataset, aes_string(x=varx, y=vary))
  )
}

And then your plotting code looks like:

pltbase <- ggplot() + geom_line(data = df1, aes(x=c1, y=c2))
pltbase + addlinetoplot(df2, varx = "c1", vary = "csq")

Results in: