Here is the link for the site where you can download the diamond-style chart for charting double weave and supplementary weft patterns.
Lately I have been using elongated ovals to chart these patterns rather than diamonds which people seem to find easier to read. Try this if you like…

you are my hero! I just got a little lap loom today, and want to try my hand at doin my own patterns. I just spent over an hour, and more different google searches then I could count tryin to find something that resembled a blank graph to help me make my own designs. It too me forever, but SO GLAD I found yours!
Wow, what a smile you have put on my face! Thanks for writing and I wish you happy charting!