Open source tools for visualizing multi-dimensional data? Besides gnuplot and ggobi, what open source tools are people using for visualizing multi-dimensional data?
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.
 A: ggobi and the R links to Ggobi are really rather good for this.   There are simpler visualisations (iPlots is very nice, also interactive, as mentioned).
But it depends whether you are doing something more specialised.   For example TreeView lets you visualise the kind of cluster dendrograms you get out of microarrays.
A: Viewpoints is useful for multi-variate data sets.
A: Python's matplotlib
A: *

*Mondrian: Exploratory data analysis with focus on large data and databases.

*iPlots: a package for the R statistical environment which provides high interaction statistical graphics, written in Java.

A: How about R with ggplot2?
Other tools that I really like:


*

*Processing

*Prefuse  

*Protovis
A: The lattice package in R.

Lattice is a powerful and elegant high-level data visualization
  system, with an emphasis on multivariate data,that is sufﬁcient for
  typical graphics needs, and is also ﬂexible enough to handle most
  nonstandard requirements.

Quick-R has a quick introduction.
A: t-SNE has many open source implementations. One of the easiest to use is probably  sklearn.manifold.TSNE. sklearn.manifold contains other manifold learning methods to plot your data to 2D:

A: Look also SCaVis data plotting library. It works on any platform since Java.
 It supports many data containers and plot styles (2D, 3D etc.)
