Index Value Plot

This plot was taken from http://waterwatch.usgs.gov/?m=real&w=plot&r=nm

This is a good example of an Index Value Plot.  This type of plot chooses a value to use as a control or as "normal".  It then measures the data against this "normal" point in a dichotomous way.  This is an example of an Index Value Plot where the control for streamflow is considered normal at the value of 4.  Anything above this point is considered wet and anything below this is considered dry.