Point 'fixed' in one direction

A and B are free points. Whenever the board is updated, e.g. after a drag event, the x-coordinate of A is set to 1 and the y-coordinate of B is set to 1. Therefore, A moves on the vertical line through x=1, B moves on the horizontal line through y=1.

The JavaScript code

var brd = JXG.JSXGraph.initBoard('jxgbox',{axis:true,boundingbox:[-1,2,2,-1]});

var A = brd.create('point', [1,0]);
var B = brd.create('point', [0,1]);

brd.on('update', function(){
        A.moveTo([1, A.Y()]);
        B.moveTo([B.X(), 1]);
});