[R] suppressing non-integer labels for plot x-axis

[Jonathan Williams]

Dear R-helpers,
I am having difficulty making R plot only integer labels on the x-axis
of a simple graph. I want to plot the median values of a score on each
of three occasions. Non-integer occasions are impossible. But, R keeps
labelling the x-axis with half-occasions, despite my attempts to stop
this using the "xaxs" and "xaxp" parameters of 'plot'.

p1=c(1,2,3); p2=c(5,15,25)
plot(p1,p2,xlab='Occasion', ylab='Score', xlim=c(1,3), ylim=c(0,30),
xaxp=c(1,3,3), xaxs='r')

Could someone let me know how to suppress the non-integer labels?

Thanks

Jonathan Williams
OPTIMA
Radcliffe Infirmary
Woodstock Road
OXFORD OX2 6HE
Tel +1865 (2)24356