BarChart is a D3.js Version 5 application.
This project is from FreeCodeCamp Project 1
I used ideas from This CodePen
Source Code

I commented every line of D3.js code, in the file BarChartUnderstandEveryLine.js.

The top few things I learned were

1. the general pattern of creating DOM elements for the vizualization web page is
   selectAll().data().enter().append();
append() is the function call that actually renders content to the screen by appending to the DOM.
2. scaleLinear().domain().range() creates a function that transforms data input into output that appears at the correct place on the web page
3. The tooltip that appears when the user hovers over a bar is the same <div>, it just appears in different places when you mouseover.