Bring in functions for graph generation

[dave-mills]

We need to bring the functions from https://github.com/stats4sd/tape-giz-analysis into this repo. They should be well commented and formatted to make clear:

• the purpose and outputs of each function;
• the requirements of the functions (do they rely on having specific dataframes present in the environment, that need to be imported from the database or local files before running the function?)

Similar to the existing functions scripts, I think we should separate out files that hold functions and files that are scripts we run. That way, we can do things like source('functions.R') and know that's just bringing in functions instead of running additional code.