2-D scatterplot question...how to assign a single numerical value to each data point


I am building and looking at scatterplots comparing two different sets of data. I would like to be able to compare the data using a single measure so that I can get an idea of how the data relates to the whole set. After I come up with a single measure for the 2 dimensional statistic I will normalize them.

There must be a common measure to do this?

If all else fails I can assign a data point in some direction relative to the cluster and find differences or something, then normalize them.

is there a better way?