As part of a consulting project, I once had data from well over 100 law firms on each of their average partner billing rates as well as their number of lawyers. Put into Excel, the data created a scatter-gram that showed a data point for each firm's rate (on one axis) matched to its corresponding size (on the other axis). Sometimes the eye readily spots clusters, open spaces or patterns. With the firms sorted by increasing numbers of lawyers, however, a trend line placed by Excel on the data points showed a quite apparent steady rise: more lawyers, higher rates.
The trend line alone helps you understand the correspondence of the data. More usefully, the software also produces a formula. That formula let you put in any size of a firm and find out its projected average partner rate. From the formula you could see that each additional 100 lawyers raised the average partner rate $13.