untranslate.js

var untranslate = function(){
  var angle = Math.atan(slope);

  var x1 = point.x - center.x;
  var y1 = point.y - center.y;

  var x2 = x1 * Math.cos(angle) - y1 * Math.sin(angle));
  var y2 = x1 * Math.sin(angle) + y1 * Math.cos(angle));

  var translated = {};
  translated.x = x2 + center.x;
  translated.y = y2 + center.y;

  return translated;
}