Question

What is the correct way to assign a ggplot2 grob to a variable if the object includes other variabels that I would like to be resolved at the time of assigning.

For example:
xpos and ypos are values that I would like resolved.
I am assigning geom_text to ptext, which I then want to add to some plots, say p1 and p2.

xpos <- .2
ypos <- .7

ptext <- geom_text(aes(x = xpos, y = ypos, label = "Some Text"))

I can add ptext to another plot and everything works as expected

## adding ptext to a plot
p1 <- qplot(rnorm(5), rnorm(5))
p1 + ptext   

However, removing (or altering) xpos & ypos gives undesired results.

rm(xpos, ypos)

p2 <- qplot(rnorm(5), rnorm(5))
p2 + ptext
# Error in eval(expr, envir, enclos) : object 'xpos' not found

What is the correct way to go about this?

Answer 1

You should put xpos and ypos in a data frame. In your example:

coords = data.frame(xpos=.2, ypos=.7)

ptext <- geom_text(data=coords, aes(x = xpos, y = ypos, label = "Some Text"))

## adding ptext to a plot
p1 <- qplot(rnorm(5), rnorm(5))
p1 + ptext   

# removing (or altering) the old coords data frame doesn't change the plot,
# because it was passed by value to geom_text
rm(coords)

p2 <- qplot(rnorm(5), rnorm(5))
p2 + ptext