Index Value Plot

http://waterwatch.usgs.gov/new/?m=real&r=nc&w=real%2Cplot
An index value plot is a visualization that compares the relativity of all values back to the index value (which is not an absolute value). The index value is plotted on the line graph as a straight line. All other values are thus plotted, and their proximity to the index value is analyzed.
This index value plot shows the average streamflow index for North Carolina for the last 45 days. The data was collected from mid February to the end of March.