This chart shows you a reasonably accurate expectation of how long your gear will last you, based on how many other people are using that gear.
Going across horizontally is a scale of Health.
Each gear item has a bar that ranges from the lowest Health of a character with that item equipped to the highest. The slightly darker region in the middle of the bar is the average with a plus or minus 5% confidence range.
The number in each bar is the count of characters that have that item equipped.
You can use the dropdown menus at the top of the page to filter your selection.
Original Coding by Rick O, Apanthrope of Bleeding Hollow-US-PvP, July 2008, WoW v2.4.3