Combining 2 datasets in a single plot in R

Question

I have two columns of data, f.delta and g.delta that I would like to produce a scatter plot of in R.

Here is how I am doing it.

plot(f.delta~x, pch=20, col="blue")
points(g.delta~x, pch=20, col="red")

The problem is this: the values of f.delta vary from 0 to -7; the values of g.delta vary from 0 to 10.

When the plot is drawn, the y axis extends from 1 to -7. So while all the f.delta points are visible, any g.delta point that has y>1 is cut-off from view.

How do I stop R from automatically setting the ylims from the data values. Have tried, unsuccessfully, various combinations of yaxt, yaxp, ylims.

Any suggestion will be greatly appreciated.

Thanks, Anjan

Answer 1

In addition to Gavin's excellent answer, I also thought I'd mention that another common idiom in these cases is to create an empty plot with the correct limits and then to fill it in using points, lines, etc.

Using Gavin's example data:

with(df,plot(range(x),range(f.delta,g.delta),type = "n"))
points(f.delta~x, data = df, pch=20, col="blue")
points(g.delta~x, data = df, pch=20, col="red")

The type = "n" causes plot to create only the empty plotting window, based on the range of x and y values we've supplied. Then we use points for both columns on this existing plot.

Answer 2

You need to tell R what the limits of the data are and pass that as argument ylim to plot() (note the argument is ylim not ylims!). Here is an example:

set.seed(1)
df <- data.frame(f.delta = runif(10, min = -7, max = 0),
                 g.delta = runif(10, min = 0, max = 10),
                 x = rnorm(10))

ylim <- with(df, range(f.delta, g.delta)) ## compute y axis limits

plot(f.delta ~ x, data = df, pch = 20, col = "blue", ylim = ylim)
points(g.delta ~ x, data = df, pch = 20, col = "red")

Which produces