Back before WYSIWYG editiors or HTML, TeX was an open source typesetting program especially revered for it’s ability to render mathematical formulae. Still going strong and expanding, even, this document (http://tex.blogoverflow.com/2013/04/how-can-i-draw-a-knot-in-tex-let-me-count-the-ways/) explains how you can draught (mathematical, but not necessarily limited to) knots with it.