Index Value Plot

http://waterwatch.usgs.gov/new/?m=pa07d&w=plot

This is an index value plot comparing a streamflow over a seven day period  compared to values recorded in previous years.  An index value plot is basically a graph that has an index value with variables plotted above or below the line.  Here the index value is defined as Normal and is at about four with the wet and dry variables plotted above and below the line.