smooth_interact <- gam(y~x0+s(x1,x2),data=gam_data)
smooth_interact_summary <- summary(smooth_interact)
print(smooth_interact_summary$s.table)
plot(smooth_interact,page=1,scheme=3)
# plot(smooth_interact,page=1,scheme=1) will give a similar plot to the vis.gam()
vis.gam(smooth_interact,view=c("x1","x2"),theta=40,n.grid=500,border=NA) 
anova(two_smooth_model,smooth_interact,test="Chisq")