var x1 = parseFloat(document.getElementById("x1").value);
var x2 = parseFloat(document.getElementById("x2").value);
var xs = 1.0 * (x2 - x1) / width;
var x1 = parseFloat(document.getElementById("x1").value);
var x2 = parseFloat(document.getElementById("x2").value);
var xs = 1.0 * (x2 - x1) / width;