Interactive diagram windows.
Haskell excels at handling data like continuous functions in a nice way, i.e. without discretising anything to finite arrays as is typically done in languages like Matlab. Instead, you can simply pass around functions or infinite data structures (or very high-resolution data that would be infeasible to handle in a strict language).
However when you want to view the data, it will eventually need to be exported out of Haskell in some finite form. The purpose of this library is to delay this discretisation as long as possible: it implements an interactive plotting window that accepts continuous/recursive data and only “flattens” it according to the specific view configuration. You can then zoom in to a shown diagram and it will automatically calculate the features more detailedly, or zoom out and discover previosly unexpected features. You don't need to worry about specifying the range and/or resolution beforehand: the program will try to find a suitable default view based on all data you're displaying, and you can always still zoom, resize or move later.
http://projects.haskell.org/diagrams are used as the “pre-rendered” type. This makes the output usable in a very wide range of applications, though at the moment only the GTK window view is implemented.