suppressing non-integer labels for plot x-axis

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?

If you want finer control over the axis labeling, it is generally best
to suppress the axis or axes in question and explicitly draw the axis
using the axis() function.

By default, the tick marks and labels will be the result of using the
pretty() function, based upon the range of values you provide. Thus you
get:

pretty(1:3)

[1] 1.0 1.5 2.0 2.5 3.0


To avoid this and have more control, do something like:

p1=c(1, 2, 3)
p2=c(5, 15, 25)

# Use 'xaxt = "n"' to suppress the x axis
plot(p1, p2, xlab = 'Occasion', ylab = 'Score',
     xlim = c(1, 3), ylim = c(0, 30), xaxt = "n")

# Now call axis to draw tick marks and labels at 1:3
axis(1, at = 1:3)

See ?pretty, ?plot.default and ?axis for more information.

HTH,

Marc Schartz