Git Draw
Allows you to draw in your GitHub heatmap
Created by Ben Friedland - http://www.bugben.com
This is a Chrome extension which will allow you to freely draw on your GitHub heatmap. You can then export your drawing to a script containing a git commit log. Once you've run and pushed this script to a new repository, your commit log will match the drawing you made.
Download the extension here:
https://chrome.google.com/webstore/detail/git-draw/aapcmdackhlfobmkcpplkjpfceihngkh?hl=en-US&gl=US
Instructional Video
My Brother, Rich Friedland, made a comprehensive video on how to use the extension.
https://www.youtube.com/watch?v=ptzDfPZ--Qk
Why not a bookmarklet?
Because CSP. GitHub's CSP policy makes a bookmarklet next to impossible. There's a workaround involving injecting code into a canvas and then executing it from there, but that feels like something that'll be fixed. A Chrome extension makes it easier. Once you're done making your drawing, just uninstall the extension.
Acknowledgements
GitFiti: https://github.com/gelstudios/gitfiti - got the idea from here, and poked around their src to see how they were writing commit messages.