[R] plot.gam

Thomas Lumley thomas at biostat.washington.edu
Fri Apr 7 19:46:07 CEST 2000

While it is relatively easy to plot the estimated curves that plot.gam
produces using predict(,type="terms") it seems to be very hard to get it

Getting the fitted function is easy. Getting the untransformed variable to
plot it against, though, is hard. 

Suppose we fit


Given 'model' how can we extract x & z?  We can't work them out from
model.frame(model), which only contains transformed versions.  Even
assuming we can guess what the name of the variable is (not trivial --
which is the variable in  ns(z,df)? In ns(df=df,z)?), we don't know where
to find it.

It seems that we would have to assume that model$call$data contains the
variables if evaluated in the calling frame. This means that plot.gam
wouldn't necessarily work if called from inside a function, or if the
original data set had been changed since the model was fitted, or if the
default na.action had been changed or various other problems.



Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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