based on Image:Biweight.png by wikipedia:en:user:Deimos 28

This image is an example of the influence function of a Tukey biweight function. It was created under R by the following code:

f = function(x, c) {

   if (abs(x) < c) {
       return(x*((1-(x/c)^2))^2)
   }
   else {
       return(0)
   }
}
x <- (-1000:1000)/100
sapply(x, f, c=3)
library(Cairo)
CairoSVG(file='biweight.svg')
plot(x, y, type="l", col=2)
dev.off()

This work has been released into the public domain by its author, Deimos 28, at the English Wikipedia project. This applies worldwide. In case this is not legally possible: Deimos 28 grants anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.