Port Charts to use Diagrams

by jbracker for haskell.org

Right now the Charts library uses Cairo as backend. Cairo can be difficult to build on platforms other then Linux. Goal of the project would be to port the Charts library to be independent of the used backend. Like that it can support to use Diagrams as backend. Diagrams supports a variety of backends (SVG, Postscript, Cairo (Optional!), Tikz, Gtk), some of which are written platform indepent. As Diagrams is easy to install on every platform supported by Haskell, so will the Charts Library built upon Diagrams. This will make it easy for Haskellers to add charting capabilities to their own applications and libraries in a manner portable across all platforms without any pain.