Here's a graph showing the charge of a battery while, well, charging it: It's a random battery charging graph, but most batteries have a similar graph like this. (gonintendo.com)
And so they came up with graphs like this — tracking antioxidant levels, and linking them to life events. (nutritionfacts.org)
Pro Tip: Another method of quantifying your skills is by using a star rating system instead of the bar graph like in the example above. (uptowork.com)