[R] A beginner's question about ggplot

[MUHC-Research]

Dear R-users,

I would have another question about the ggplot() function in the ggplot2
package.

All the examples I've read so far in the documentation make use of a single
neatly formatted data.frame. However, sometimes, one may be interested in
plotting on the same grid information or objects derived from two totally
different datasets and customize both displays. I still cannot tell how this
can be done using ggplot().

Here's an example.

###############
## A very simple data.frame;

my.data = data.frame(X1 =
as.factor(rep(1:2,c(4,4))),X2=c(4,3,5,2,6,2,3,5),X3=c(1:3,2,2:4,5)) ;

## Let's say I want to add the X^2 line to the plot;

squared = data.frame(X=1:12,Y=((1:12)/2)^2) ;

## A scatterplot for my.data ;

p = ggplot(my.data,aes(x=X2,y=X3,group=X1)) ;
p = p+geom_point(aes(colour=X1)) ; 

#############

How can "squared" be added to the plot? At first, I used

p+geom_line(data=squared,aes(x=X,y=Y,group=1,colour="green")) ;

but the plotted line is always blue! In fact, I can replace colour by any
character value and I will still get a blue line.

Although I may be wrong, I think this is pretty straightforward. Can anyone
give me a pointer as to how we can add arbitrary curves to a ggplot graph
and then customize them? A bit later, I'll have to overlay histograms
derived from totally different datasets and, if possible, I'd like to use
the ggplot2 library for that too, hence the importance of understanding how
ggplot objects can be mixed.

Thanks a lot,

Luc
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