Research charting libararies for Flutter

[SarahBornais]

Motivation

A key part of our app is data visualization - let's find the best way to implement this!

Task

Add a folder called research in our GitHub repository. In this folder, add a markdown file called data_visualization_research.md and in it outline what options you considered, the pros and cons of each of them, and what your main recommendation would be and why.

When doing your research, keep in mind that we need something fairly customizable (different kinds of charts and preferably a way to alter the color scheme/theme). Having good documentation is also important.