使用 coplot 调整绘图边距

Question

How does one change the upper margin of the plotting region when using coplot to add a title? Changing the par values doesn't seem to have an effect. I want to increase the upper margin to put a title there, but changing par(mar) isn't working as I expected it to. Here is an example,

opar <- par(mar=c(5.1, 4.1, 20, 1))
coplot(Sepal.Length ~ Sepal.Width | Species, data=iris, columns=3,
  bar.bg=c(fac="light green"), panel=panel.smooth)   # mar=c(5.1, 4.1, 10, 1)
title("test", outer=TRUE)
par(opar)

Answer 1

You can move it down slightly so that it is not "off the edge":

opar <- par(mar=c(5.1, 4.1, 20, 1))
coplot(Sepal.Length ~ Sepal.Width | Species, data=iris, columns=3,
  bar.bg=c(fac="light green"), panel=panel.smooth)   # mar=c(5.1, 4.1, 10, 1)
title("test", line =-1, outer=TRUE)
par(opar)

Efforts to affect the space on the top margin failed with par() and I suspect that this is one of the plot methods that hard-codes margins. In fact, here's the code in coplot:

 mar <- if (have.b) 
        rep.int(0, 4)
    else c(0.5, 0, 0.5, 0)
    oma <- c(5, 6, 5, 4)
    if (have.b) {
        oma[2L] <- 5
        if (!b.is.fac) 
            oma[4L] <- 5
    }
    if (a.is.fac && show.given[1L]) 
        oma[3L] <- oma[3L] - 1

 opar <- par(mfrow = c(total.rows, total.columns), oma = oma, 
        mar = mar, xaxs = "r", yaxs = "r")

Answer 2

As @42 mentions it looks like the code of coplot contains a hard coded value of mar : https://github.com/wch/r-source/blob/bfe73ecd848198cb9b68427cec7e70c40f96bd72/src/library/graphics/R/coplot.R#L214

An alternative solution is to copy/paste the coplot code into your own script and change the value of mar in the function, but that seems like overkill if the solution by @42 does the job for you.