r multiple graphs on a single jpeg - r

Question

I have run into the following problem and I would appreciate if someone could give me some input.
I would like to export multiple figures to a single jpeg file. I first create a graphics lattice and then I export. My main issue is that it works with the pdf and not the jpeg. Any ideas?
Thank you
#set the windows of the frames
par(mfcol=c(3,2))
#create the jpeg file
jpeg(filename=names(a1),".jpg",sep=""),
quality=100,
width=1024,
height=768)
#plot 1
plot(a1,b1)
#plot 2
plot(a1,b2)
#plot 3
plot(a1,b3)
#plot 4
plot(a2, c1)
#plot 5
plot(a2, c2)
#plot 6
plot(a2, c3)
#dev.off shuts down the specified (by default the current) graphical device
#here it passes the picture to the file
dev.off()

Answer 1

It is not clear whether you want multiple 1024x768 images in a single jpeg file - which doesn't make sense - are whether you want a single jpeg image containing the 6 plots.
As I said, JPEGs are not a multi-page format, unlike a PDF. Hence you can get R to export to multiple JPEG files but not have all the separate figures in one JPEG.
R's devices allow for wildcards in the filenames, so if you want the six plots exported to files foo001.jpeg, foo002.jpeg, foo00x.jpeg then you can use the following
jpeg(filename = "foo%03d.jpeg", ....)
.... # plotting commands here
dev.off()
What happens if you do multiple plots without the wildcard/placeholder is document in say ?jpeg:
If you plot more than one page on one of these devices and do not
include something like ‘%d’ for the sequence number in ‘file’, the
file will contain the last page plotted.
Devices that handle multiple pages because the underlying file format allows it can take multiple plots into the single file as there is makes sense, e.g. pdf() and postscript(). Those devices have argument onefile which can be used to indicate if multiple plots in a single file are required.
However, the par(mfcol=c(3,2)) makes me think you want a 3x2 set of plots in the same device region. That is allowed, but you need to call par() after you open the jpeg() device, not before. What your code, as shown, does is split the active device into 3x2 plotting regions and then opens a new device which picks up the default parameters, not the ones you set on the device active before you called jpeg(). This is illustrated below:
> plot(1:10)
> dev.cur()
X11cairo
2
> op <- par(mfrow = c(3,2))
> jpeg("~/foo.jpg")
> par("mfrow")
[1] 1 1
> dev.off()
X11cairo
2
> par("mfrow")
[1] 3 2
Hence you want perhaps wanted something like:
jpeg(filename=names(a1),".jpg",sep=""), quality=100,
width=1024, height=768)
op <- par(mfcol=c(3,2))
#plot 1
plot(a1,b1)
#plot 2
plot(a1,b2)
#plot 3
plot(a1,b3)
#plot 4
plot(a2, c1)
#plot 5
plot(a2, c2)
#plot 6
plot(a2, c3)
par(op)
dev.off()
?

Answer 2

The corrected code
#data
a1<-seq(1,20,by=1)
b1<-seq(31,50,by=1)
b2<-seq(51,70,by=1)
b3<-seq(71,90,by=1)
#create the jpeg file
jpeg(filename="a.jpg",
quality=100,
width=1024,
height=768)
#set the create the frames
par(mfcol=c(3,1))
#plot the graphs
plot(a1,b1)
plot(a1,b2)
plot(a1,b3)
#par(op)
#dev.off shuts down the specified (by default the current) graphical device
#here it passes the picture to the file
dev.off()