Adding points after the fact with ggplot2; user defined function

Question

I believe the answer to this is that I cannot, but rather than give in utterly to depraved desperation, I will turn to this lovely community.

How can I add points (or any additional layer) to a ggplot after already plotting it? Generally I would save the plot to a variable and then just tack on + geom_point(...), but I am trying to include this in a function I am writing. I would like the function to make a new plot if plot=T, and add points to the existing plot if plot=F. I can do this with the basic plotting package:

fun <- function(df,plot=TRUE,...) {
...
if (!plot) { points(dYdX~Time.Dec,data=df2,col=col) }
else { plot(dYdX~Time.Dec,data=df2,...) }}

I would like to run this function numerous times with different dataframes, resulting in a plot with multiple series plotted.

For example,

fun(df.a,plot=T)
fun(df.b,plot=F)
fun(df.c,plot=F)
fun(df.d,plot=F)

The problem is that because functions in R don't have side-effects, I cannot access the plot made in the first command. I cannot save the plot to -> p, and then recall p in the later functions. At least, I don't think I can.

why don't you pass the plot variable to your function which then either returns the appended plot or a new plot depending on plot=T/F ? — mts
– mts, Commented Jul 10, 2015 at 8:10
Maybe! How do I pass the plot variable to my function if I am defining the plot variable in a function? Is there a way for a variable defined within a function to be used outside that function? — Unrelated
– Unrelated, Commented Jul 10, 2015 at 8:14
return it. if you want to return more than one object from your function to your working environment, stuff everything into a list. in the first iteration just pass an empty plot to your function — mts
– mts, Commented Jul 10, 2015 at 8:15

mts · Accepted Answer · 2015-07-10 08:37:30Z

1

have a ggplot plot object be returned from your function that you can feed to your next function call like this:

ggfun = function(df, oldplot, plot=T){
  ...
  if(plot){
    outplot = ggplot(df, ...) + geom_point(df, ...)
  }else{
    outplot = oldplot + geom_point(data=df, ...)
  }
  print(outplot)
  return(outplot)
}

remember to assign the plot object returned to a variable:

cur.plot = ggfun(...)

answered Jul 10, 2015 at 8:37

mts

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