Index Value Plot

http://www.skepticalscience.com/What-causes-short-term-changes-in-ocean-heat.html

An index value plot uses index value instead of absolute numbers. In the index value plot above, the index is a zero value and the departures in the index are plotted relative to it. Positive increases (blue) indicate La Nina conditions and Negative decreases (red) indicate El Nino conditions. The line going through the graph represents the 12 month average southern oscillation index.