Visualizing a spline basis

Question

Textbooks typically have nice example plots of of the basis for uniform splines when they're explaining the topic. Something like a row of little triangles for a linear spline, or a row of little humps for a cubic spline.

This is a typical example:

http://support.sas.com/documentation/cdl/en/statug/63033/HTML/default/viewer.htm#statug_introcom_a0000000525.htm

I'm wondering if there is an easy way to generate a plot of the spline basis using standard R functions (like bs or ns). I guess there's some simple piece of matrix arithmetic combined with a trivial R program which will spit out a pretty plots of a spline basis in an elegant way. I just can't think of it!

Answer 1

Try this, as an example for B-splines:

x <- seq(0, 1, by=0.001)
spl <- bs(x,df=6)
plot(spl[,1]~x, ylim=c(0,max(spl)), type='l', lwd=2, col=1, 
     xlab="Cubic B-spline basis", ylab="")
for (j in 2:ncol(spl)) lines(spl[,j]~x, lwd=2, col=j)

Giving this: