R: "InvalidArgument `-delay' with animation and ggplot

Question

I'm trying to make an animated plot in R from a large data set (from a cyclic scientific experiment), to visualise the change in two variables over time. I'm using the animation library with just simply:

saveGIF(
    for(i in 1:100){
    mygraph(i)
}, interval = 0.1, ani.width = 640, ani.height = 480)

where mygraph(i) just plots the graph for the cycle i. If I use plot() to make the graph, then it works perfectly fine, but if I instead use ggplot (which I would like to do as I eventually want to use this to make more complex plots), then it doesn't work and I get the following output:

Executing: 
'convert' -loop 0 -delay 'animation.gif'
convert: InvalidArgument `-delay': animation.gif @ error/convert.c/ConvertImageCommand/1161.
an error occurred in the conversion... see Notes in ?im.convert
[1] FALSE 

I'm very new to R so I'm a bit stuck, and I've not worked out a solution from looking at ?im.convert or from searching around. Any suggestions would be enormously appreciated...

Example with dummy data as requested:

library(animation)
library(ggplot2)

x <- 1:20
y <- 21:40
z <- c(1,1,1,1,1,2,2,2,2,2,3,3,3,3,3,4,4,4,4,4)
data <- data.frame(x,y,z)

mygraph <- function(i) {
  plot(data$x[data$z == i], 
       data$y[data$z == i], 
       title(title))
}

saveGIF(
  for(i in 1:4){
    title <- paste("Cycle", i, sep=" ")
    mygraph(i)
  }, interval = 0.5, ani.width = 640, ani.height = 480)

This works, but if the function mygraph is instead:

mygraph <- function(i) {
  ggplot() +
    geom_point(aes(x=data$x[data$z == i], y=data$x[data$z == i]))
}

...then it gives me the error as stated above.

Answer 1

This appears to work if you wrap ggplot in a print() statement, e.g.

mygraph <- function(i) {
  g <- ggplot() +
    geom_point(aes(x=data$x[data$z == i], y=data$x[data$z == i]))
  print(g)
}

This is a variant of R-FAQ 7.22, Why do lattice/trellis graphics not work?