Add legends to the arrow chart and scatter plot

[sarahfossheim]

The arrow chart and scatter plot use color coding that's not explained. A legend should be added.

For the arrow chart it's a range, and the value and meaning of the values should be explained. What does it mean for a technology to be blue vs white vs red.

The scatter plot uses the colors to indicate what type of tool it is (front-end frameworks, data layer, testing, etc), should add which color maps to which category.