[Rd] Wishlist for plot.lm() (PR#13560)

[john.maindonald at anu.edu.au]

Full_Name: John Maindonald
Version: R-2.8.1
OS: MacOS X 10.5.6
Submission from: (NULL) (203.173.3.75)


The following code demonstrates an annoyance with plot.lm():

library(DAAGxtras)
x11(width=3.75, height=4)
nihills.lm <- lm(log(time) ~ log(dist) + log(climb), data = nihills)
plot(nihills.lm, which=5)

OR try the following
xy <- data.frame(x=c(3,1:5), y=c(-2, 1:5))
plot(lm(y ~ x, data=xy), which=5)

The "Cook's distance" text overplots the label for the point with the smallest
residual.  This is an issue when the size of the plot is much less than the
default, and the pointsize is not reduced proportionately.


I suggest the following:
     xx <- hii
     xx[xx >= 1] <- NA
## Insert new code
     fracht <- (1.25*par()$cin[2])/par()$pin[2]
     ylim[1] <- ylim[1] - diff(ylim)*max(0, fracht-0.04)
## End insert new code
     plot(xx, rsp, xlim = c(0, max(xx, na.rm = TRUE)),
          ylim = ylim, main = main, xlab = "Leverage",
          ylab = ylab5, type = "n", ...)

Then, about 15 lines further down, replace
       legend("bottomleft", legend = "Cook's distance",
              lty = 2, col = 2, bty = "n")

by
       legend("bottomleft", legend = "Cook's distance",
              lty = 2, col = 2, text.col=2, bty = "n", y.intersp=0.5)
         # This changes the legend color to agree with the line color

Another possibility, suggested by John Fox, is to replace the caption by "Cook's
distance contours", and omit the legend entirely.  

Both John Fox and myself are comfortable with either of these fixes.

Test the changes with:
x11()
nihills.lm <- lm(log(time) ~ log(dist) + log(climb), data = nihills)
plot(nihills.lm, which=5)
xy <- data.frame(x=c(3,1:5), y=c(-2, 1:5))
plot(lm(y ~ x, data=xy), which=5)
x11(width=3.75, height=4)
plot(nihills.lm, which=5)
plot(lm(y ~ x, data=xy), which=5)