[R] How to put x-axis labels vertically?

[Peng Yu]

image(array,axes=F)
axis(side=1,at=seq(from=par()$usr[1],to=par()$usr[2],length=size),labels=some_labels)

I have the above code to plot a heatmap. But I want to make the labels
on the x-axis vertical. Could somebody let me know how to do it?