stair-step plot

Hello
Is it possible to map a plot with horizontal lines like in the step-plot,
but without the vertical lines?

Not in the basic plot function, but you can write your own fairly 
easily, using segments().  For example:

x <- y <- 1:10
plot(x,y, type='n')
segments(x[-10], y[-10], x[-1], y[-10])

Duncan Murdoch