If you have a GitHub account, then you will be able to see a section called Public contributions on your profile page. Your contribution status for the past year will be showed on this section, it's called contribution calendar.
This calendar has 7 rows and 53 columns. Each cell on the grid represents your contribution status for one day, if the contribution is different, the cell color will be different. There are 5 colors in total : white, yellow, light green,green and deep green.
With this, some people start to show their creativity, they start to draw on this grid. Let's check some interesting drawings here.
1. Hello
2. GitHub
4. WILL
5. Love
6. Awesome
7. Key
8. Frog
You can use the library here to draw the graph on your contribution calendar.
Source : http://blog.jobbole.com/48809/
#7 isn't a key...