Reading and interpreting a WPM waterfall chart

I was digging about for some info on the waterfall chart and came across a description from another site. Although the presentation is slightly different from WPM, it makes a good intro.

http://www.webperformancetoday.com/2014/03/18/waterfalls-101-how-to-use-a-waterfall-chart-to-diagnose-performance-pains/

Gaps in charts are explained well at

javascript - Pingdom Website Speed Test - Waterfall GAP - Stack Overflow