Gnuplot is more or less a basic plotting package.
Ggobi can do a number of nifty things, such as:
- animate data along a dimension or among discrete collections
- animate linear combinations varying the coefficients
- compute principal components and other transformations
- visualize and rotate 3 dimensional data clusters
- use colors to represent a different dimension
What other useful approaches are based in open source and thus freely reusable or customizable?
Please provide a brief description of the package's abilities in the answer.