Different labels for the x-axis

i have this data and plot

mydata <- data.frame(a=c(1:5),b=c(6:10),c=c(11:15),e=c(16:20))
plot <- stripchart(mydata, method="jitter", vertical=T,main='plot',pch=19)

I would like to subset the x axis into two labels named 'a+b' and 'c+d' labels

Thanks in advance

Solution

In your case you can simply use mtext on side 1:

mydata <- data.frame(a=c(1:5),b=c(6:10),c=c(11:15),d=c(16:20))
plot <- stripchart(mydata, method="jitter", vertical=T,main='plot',pch=19)
mtext(c('a+b','c+d'),side=1,line=3,at=c(1.5,3.5))

Argument line is to set up the vertical position and at the position on the x-axis.

Edit: To add a distance between the two groups, you can do like this (there may be a cleaner way to do that but that's the only one I can think of from the top of my head):

mydata <- data.frame(a=c(1:5),b=c(6:10),c=c(11:15),d=c(16:20))
plot <- stripchart(mydata, method="jitter", vertical=T, main='plot',pch=19, 
                   at=c(1,2,4,5),xlim=c(0,6))
mtext(c('a+b','c+d'),1,line=3,at=c(1.5,4.5))

Argument at of stripchart is the one to fiddle with, but you then have to modify the plot limits (xlim) and the x-value at which you write the axis label (in mtext).