For something nice and "good looking" in Javascript you might check PaperJS

http://paperjs.org/examples/

I haven't really used it but it looks well made and it shares many of Processing's good features: it's free, programs tend to be small and rather readable, and it is JS-based so it should work on various platforms :)

Thanks Andrea. I appreciate very much your answer. I had heard about it but your answer made me focus on it. It looks great!.

See touchdevelop.

Hi Tiago,

cool stuff! I was looking for something to program my android tablet without an external computer, and so far I played a bit with this Catroid: https://pocketcode.org/catroid/index/1

but touchdevelop seems nicer! I'll check it asap :)

I am a big fan of D3 personally. As its name (Data Driven Documents) implies, its API is very cleverly designed to convert your data to SVG as painlessly/straighforwardly as possible. Since it is pure Javascript, you can add other HTML elements (i.e. buttons ,forms) to your visualization and then allow the user to explore your data visualization in various ways. Just check the examples in its Gallery page (linked below) and you can appreciate yourself its power and breadth. Scroll down to check interesting examples that don't have a thumbnail image.

You may also want to check JIT (JavaScript InfoVis Toolkit -http://philogb.github.io)/jit/). Compared to D3 , its API allows you more easily/directly create some shapes or layouts. Its library and utility functions is not as extensive as D3 but if you ask the question for a one-time project including a TreeMap or Graph then it may be easier to learn and implement a solution. Although JIT looks impressing and promising, I am also not sure how the future looks like for it. It may become an abandoned project in a couple of years but you don't need to have such worries for D3.

To sum up, I'd strongly suggest investing time to learn D3 if you are looking for a long term, inspiring and extensive visualization library.

https://github.com/mbostock/d3/wiki/Gallery

If you're comfortable using Processing, I'd recommend JavaFX (http://docs.oracle.com/javafx/index.html) as your next step. It will allow you to draw pictures as easily as you did with Processing and you'll have the benefits of an easy to use UI controls. Also, JavaFX is base on a scene graph, so some types of visualizations are easier to create. We have started using it in the lab and it is has been great!