Polygon through ordered set of gliders

The underlying JavaScript code

    board = JXG.JSXGraph.initBoard('jxgbox', {axis:true, boundingbox: [-5, 5, 7, -5]});
    board.suspendUpdate();
    var p = [];
    p[0] = board.createElement('point', [-1,2], {size:2,name:''});
    p[1] = board.createElement('point', [0,-2], {size:2,name:''});
    p[2] = board.createElement('point', [2,1], {size:2,name:''});
    
    graph = board.createElement('curve', JXG.Math.Numerics.Neville(p),{strokeWidth:5,strokeOpacity:0.5});

    gliders = [];
    gliders[0] = board.createElement('glider', [-1,0,graph],{size:4});
    gliders[1] = board.createElement('glider', [-0.5,-2,graph],{size:4});
    gliders[2] = board.createElement('glider', [1,-3,graph],{size:4});
    gliders[3] = board.createElement('glider', [1.5,-1,graph],{size:4});
    
    board.createElement('polygon',gliders,{strokeColor:'red'});
    
    board.updateConditions = function() {
        var i;
        for (i=1;i<gliders.length;i++) {
            if (gliders[i].position<gliders[i-1].position) {
                gliders[i].coords = new JXG.Coords(JXG.COORDS_BY_USER, gliders[i-1].coords.usrCoords, board); 
                gliders[i].update();
            }
        };
    }
    board.unsuspendUpdate();