How do I draw gridlines using abline() that are behind the data?

Question

When I draw grid lines on a plot using abline() the grid lines are drawn over the data.

Is there a way to draw the abline() lines behind

Answer 1

Plot first with type="n" to establish coordinates. Then put in the grid lines, then plot again with your regular plot type:

plot(x, y, col = 'red', type = 'n', lwd = 3, pch = 15)
abline(h = seq(0, 10, .5), col = 'lightgray', lty = 3)
abline(v = seq(0, 10, .5), col = 'lightgray', lty = 3)
par(new=TRUE)
plot(x, y, col = 'red', type = 'o', lwd = 3, pch = 15)

I admit that I have always thought the name for that par parameter was "backwards."

Answer 2

The panel.first argument of plot() can take a list or vector of functions so you can put your abline() calls in there.

plot(1:4, panel.first = 
       c(abline(h = 1:4, lty = 2, col = 'grey') 
        ,abline(v = 1:4, lty = 2, col = 'grey')))

Answer 3

Use plot() to set up the plotting window, but use type = "n" to not plot any data. Then do your abline() calls, or use grid(), and then plot the data using whatever low-level function is appropriate (here points() is fine).

x <- seq(0, 10)
y <- x
plot(x, y, type = "n")
abline(h = seq(0, 10, .5), col = 'lightgray', lty = 3)
abline(v = seq(0, 10, .5), col = 'lightgray', lty = 3)
points(x, y, col = 'red', type = 'o', lwd = 3, pch = 15)

or

## using `grid()`
plot(x, y, type = "n")
grid()
points(x, y, col = 'red', type = 'o', lwd = 3, pch = 15)

See ?grid for details of how to specify the grid as per your abline() version.

Answer 4

Another way of creating grid lines is to set tck=1 when plotting or in the axis function (you may still want to plot the points using points after creating the grid lines.