As the title says, which programming language would you agree had the best libraries for visualisation (graphs, 3d models, charts, networks, animations, etc)?
Prefer languages with libraries that have more visulatizaton features than say, ease of using the libraries.
For certain highly formalized types of diagrams - for example flow charts -, PlantUML is great, and since it uses a textual language, it could be wired to a lot of different programs.
MetaPost ist similar but a bit lower level. It can generate very complex diagrams for arbitrary content.For example, I used it once to draw and publish a globe-like 3D sphere with fields depicting different spherical probability values in it.
Racket, a Scheme descendent, is very nice for elementary graphics - it can output pictures in the REPL of its integrated IDE, DrRacket.
And then there is Python with Matplotlib. It is made for visualization of numerical data.
For 3D data, there is VTK.
And TeX/LaTeX with various graphing packages.