Index Value Plot

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

Index value plots are helpful in seeing the discrepancies and inconsistencies in data along with seeing the general trend of the data. In this case, the index value is 0, index value plots show the positive and negative values according to that index value. As seen in this plot, la Nina conditions and el Nino conditions differ, on opposite sides of the 0 of the Southern Oscillation index.