Catmull-Rom splines
From JSXGraph Wiki
The underlying JavaScript code
var brd = JXG.JSXGraph.initBoard('box', {axis:true, boundingbox: [-5, 5, 5, -5]}),
p = [], c, i;
for (i=0;i<5;i++) {
p.push(brd.create('point', [(Math.random()-0.5)*7,(Math.random()-0.5)*7],{withLabel:true}));
}
c = brd.create('curve', JXG.Math.Numerics.CatmullRomSpline(p), {strokeWidth:3});