CourtListener released an application developed by the University of Baltimore School of Law and the Free Law Project that allows users to visualize the historical development of the law for free. The application is "Supreme Court Citation Networks."
Each opinion released by the Supreme Court is underpinned by references to other cases, precedent. Precedent develops into legal principles, and lawyers and legal scholars study precedent to better understand legal principles. CourtListener lets you take two cases and map the citations they have in common. It also allows the user to visualize whether the cases cited were "liberal" or "conservative" decisions.
Account registration is required to use the free app. Once in the app, creating Supreme Court Citation Networks is simple. Enter two cases and the app does all the work for you. Here is an example I did mapping interracial marriage to gay marriage, the first shows the degrees of separation and the second shows political ideology (click the images to enlarge them):
Interested in learning how lawyers and scholars can use visualization to tell a story? Watch this video on Mapping Supreme Court Doctrine: Civil Pleading: