Improve usability and readability of node size legend

[AlexanderHYang]

Improve usability and readability of node size legend (that’s another thing we didn’t get to improve), so something similar to (no need to be identical): http://friendlycities.gatech.edu/data-interactive/

[AlexanderHYang]

A relevant implementation https://www.d3-graph-gallery.com/graph/bubble_legend.html