[R] How is the line in av.plot calculated?

[Duncan Murdoch]

On 2/16/2007 12:57 PM, Tom La Bone wrote:
> I suspect that the line in the added variable plot (library car) is a SLR of
> the residuals, but I can't seem to find this written anywhere.  Can someone
> confirm this?  Thanks.

You can look at the source:

 > av.plot
function (model, ...)
{
     UseMethod("av.plot")
}
<environment: namespace:car>

So the answer will depend on what kind of model you want.  The method 
for lm objects would be in av.plot.lm, which is not directly visible:

 > av.plot.lm
Error: object "av.plot.lm" not found

however getAnywhere finds it:

 > getAnywhere("av.plot.lm")
A single object matching 'av.plot.lm' was found
It was found in the following places
   registered S3 method for av.plot from namespace car
   namespace:car
with value

function (model, variable, labels = 
names(residuals(model)[!is.na(residuals(model))]),
     identify.points = TRUE, las = par("las"), col = palette()[2],
     pch = 1, lwd = 2, main = "Added-Variable Plot", ...)
{
     variable <- if (is.character(variable) & 1 == length(variable))
         variable
     else deparse(substitute(variable))
     mod.mat <- model.matrix(model)
     var.names <- colnames(mod.mat)
     var <- which(variable == var.names)
     if (0 == length(var))
         stop(paste(variable, "is not a column of the model matrix."))
     response <- response(model)
     responseName <- responseName(model)
     if (is.null(weights(model)))
         wt <- rep(1, length(response))
     else wt <- weights(model)
     res <- lsfit(mod.mat[, -var], cbind(mod.mat[, var], response),
         wt = wt, intercept = FALSE)$residuals
     plot(res[, 1], res[, 2], xlab = paste(var.names[var], "| others"),
         ylab = paste(responseName, " | others"), main = main,
         las = las, col = col, pch = pch)
     abline(lsfit(res[, 1], res[, 2], wt = wt), col = col, lwd = lwd)
     if (identify.points)
         identify(res[, 1], res[, 2], labels)
}
<environment: namespace:car>
 >

from which I would conclude that your description is correct:  the line 
is drawn in the abline() call near the bottom.

Duncan Murdoch