When you delete or move a message, most e-mail programs ("clients") simply hide the message and mark it as ready for physical deletion later on. The "deleted" messages are still stored in the folder - even emptying the Trash does not remove them. The hidden messages are not physically removed until the folder is compacted (not to be confused with compressing a file). If you don't compact your mail folders periodically, they can grow very large.
The Google Public Data Explorer makes large datasets easy to explore, visualize and communicate. As the charts and maps animate over time, the changes in the world become easier to understand. You don't have to be a data expert to navigate between different views, make your own comparisons, and share your findings.