Index Value Plot

http://waterwatch.usgs.gov/new/?m=real&w=plot&r=nm

This map is an example of an index value plot. Here, we can look at variations in a particular situation. Here, this comes from the USGS where they are looking at streamflow, and in this case, of New Mexico. We can see the standard line, normal and the etchings in the line show variations from the normal, so whether it was wet or dry that year. These types of visualizations can be very handy at looking at changes in something that occur over a period of years.