Animation, adding geom - r

Question

I want to create some kind of animation with ggplot2 but it doesn't work as I want to. Here is a minimal example.
print(p <- qplot(c(1, 2),c(1, 1))+geom_point())
print(p <- p + geom_point(aes(c(1, 2),c(2, 2)))
print(p <- p + geom_point(aes(c(1, 2),c(3, 3)))
Adding extra points by hand is no problem. But now I want to do it in some loop to get an animation.
for(i in 4:10){
Sys.sleep(.3)
print(p <- p + geom_point(aes(c(1, ),c(i, i))))
}
But now only the new points added are shown, and points of the previous iterations are deleted. I want the old ones still to be visible. How can I do this?

Answer 1

Either of these will do what you want, I think.
# create df dynamically
for (i in 1:10) {
df <- data.frame(x=rep(1:2,i),y=rep(1:i,each=2))
Sys.sleep(0.3)
print(ggplot(df, aes(x,y))+geom_point() + ylim(0,10))
}
# create df at the beginning, then subset in the loop
df <- data.frame(x=rep(1:2,10), y=rep(1:10,each=2))
for (i in 1:10) {
Sys.sleep(0.3)
print(ggplot(df[1:(2*i),], aes(x,y))+geom_point() +ylim(0,10))
}
Also, your code will cause the y-axis limits to change for each plot. Using ylim(...) keeps all the plots on the same scale.
EDIT Response to OP's comment.
One way to create animations is using the animations package. Here's an example.
library(ggplot2)
library(animation)
ani.record(reset = TRUE) # clear history before recording
df <- data.frame(x=rep(1:2,10), y=rep(1:10,each=2))
for (i in 1:10) {
plot(ggplot(df[1:(2*i),], aes(x,y))+geom_point() +ylim(0,10))
ani.record() # record the current frame
}
## now we can replay it, with an appropriate pause between frames
oopts = ani.options(interval = 0.5)
ani.replay()
This will "record" each frame (using ani.record(...)) and then play it back at the end using ani.replay(...). Read the documentation for more details.
Regarding the question about why your code fails, the simple answer is: "this is not the way ggplot is designed to be used." The more complicated answer is this: ggplot is based on a framework which expects you to identify a default dataset as a data frame, and then associate (map) various aspects of the graph (aesthetics) with columns in the data frame. So if you have a data frame df with columns A and B, and you want to plot B vs. A, you would write:
ggplot(data=df, aes(x=A, y=B)) + geom_point()
This code identifies df as the dataset, and maps the aesthetic x (the horizontal axis) with column A and y with column B. Taking advantage of the default order of the arguments, you could also write:
ggplot(df, aes(A,B)) + geom_point()
It is possible to specify things other than column names in aes(...) but this can and often does lead to unexpected (even bizarre) results. Don't do it!.
The reason, basically, is that ggplot does not evaluate the arguments to aes(...) immediately, but rather stores them as expressions in a ggplot object, and evaluates them when you plot or print that object. This is why, for example, you can add layers to a plot and ggplot is able to dynamically rescale the x- and y-limits, something that does not work with plot(...) in base R.