Affine ratio

The more popular name seems to be 'simple ratio'.

The underlying JavaScript code

JXG.Options.label.autoPosition = true;
JXG.Options.text.fontSize = 24;

var board = JXG.JSXGraph.initBoard("jxgbox", {boundingbox: [-5,5,5,-5], showCopyright:false, showNavigation:false});

var p = board.create('point', [-3,0], {name: 'p'});
var q = board.create('point', [3,0], {name: 'q'});
var pq = board.create('segment', [p, q]);
var t = board.create('glider', [-2, 0, pq], {name: 't'});
var txt = board.create('text', [1, 2, function() { return "λ = " + t.position.toFixed(2); }]);