Draw grid lines on specific values in xyplot

Question

I have a xyplot and I want to draw grid lines on the 0 values.

How this can be done?

Answer 1

If you're using package lattice (which is implied with xyplot), you can use panel.abline to draw lines over labeled ticks.

my.df <- data.frame(a = runif(10, min = -1, max = 1), b = runif(10, min = -1, max = 1))
my.plot <- xyplot(b ~ a, data = my.df)
update(my.plot, panel = function(...) {
            panel.abline(h = 0, v = 0, lty = "dotted", col = "light grey")
            panel.xyplot(...)
        })