An example of the vis.js dynamic, browser based visualization library. More info @ https://ww3.arb.ca.gov/ei/tools/lib/vis/docs/. One can replicate the desktop functionality on a mobile device as well.

On a desktop computer, click once on one of the bubbles first, and then drag it around. One can click on the screen and move it around. Scroll the mouse wheel up & down to resize the displayed objects. A red node is the highest level; then blue, and finally green.