The scripts
This requires two external scripts, my 'get' method variable reading script to interpret the variables passed by the form, and my MultiGraph creator script to draw the graph (basically a carefully customised scatter graph). The rest of the script is held in the source code for this page. You may copy this page and use it on your site. You may even reword and restyle it, subject to my usual terms and conditions.
To download the script(s), see the script license, and check details like browser compatibility, use the links on the navigation panel at the top of this page.
Calculator
Try these examples: