Last summer I needed a diameter chart in a pinch so I made one using SVG. I have added the svg source-file and a jpeg rendering to mediterraneanceramics.org. Nothing too fancy but perhaps useful if you don't have a chart at hand when the time comes to describe your sherds.
If you look at the SVG code itself, you'll see that I have placed the file in the public domain. I've used the language found on Wikimedia Commons, which I think should be sufficient to set this free. This means that you can make changes or do whatever you want to the file. If so inclined, send me any improvements with a statement that your work is in the public domain as well.