The beauty of working on this short project was in building it all from scratch and finding out some amazing things about git, Bash and GitHub along the way. For example, git and Github allow to make commits in the future and, surprisingly, GitHub displays those commits in the contribution graph too.
The script to use (or better not) is display_grid.sh . Others were useful to test some ideas before incorporating them into the main script.

Usage:
./display_grid.sh -d "2020-01-01" -n 366 -s random -c 10

Options:

-h help;
-d specify date to start from in the format YYYY-MM-DD; default: "2020-01-01";
-c specify number of commits per day; default: 5; maximum; 59;
if used with flag "-s random", will specify the max value of the range;
-n number of consecutive days to make commits for; default: 5;
-s style of the grid colour. Style options:  
  fixed - same colour for every square;  
  random - in the range from 1 and number specified with flag "-n"; 

Testing commits for the whole 2018 year: