The R Book

Three-Dimensional Graphic Output from gam

Here is an example by Simon Wood which shows the kind of three-dimensional graphics that can be obtained from gam using vis.gam when there are two continuous explanatory variables. Note that in this example the smother works on both variables together, y~s(x,z):

par(mfrow=c(1,1))
test1<-function(x,z,sx=0.3,sz=0.4)
  {(pi**sx*sz)*(1.2*exp(-(x-0.2)^2/sx^2-(z-0.3)^2/sz^2)+
  0.8*exp(-(x-0.7)^2/sx^2-(z-0.8)^2/sz^2))
  }
  n<-500
x<-runif(n);z-runif(n);
y<-test1(x,z)+rnorm(n)*0.1
b4<-gam(y~s(x,z))
vis.gam(b4)

Note also that the vertical scale of the graph is the linear predictor, not the response.