Python is my go-to language, so I use matplotlib frequently. But I think R's ggplot2 is easily the best visualization library out there.

Bokeh is supposed to be a ggplot2 clone for python, but it's still in the very early stages.

I’d strongly recommend matplotlib. It has a matlab like procedural interface that makes it very easy to pickup if you’re coming from matlab.

Additionally, it’s wonderful to be able to “dive under the hood” if you need to create a special type of visualization: i.e. the internal model is easy to understand and highly customizable. This has enabled me to create several awesome plots that would have been very difficult, if not impossible to create with any other plotting program or library.

I completely agree about the breadth of matplotlib, though the difficulty can be looking for examples. I've often found the examples on the matplotlib website to be horribly out of date compared to features added in recent releases. Are the plots you've done available online / Any recommended blogs for interesting examples of plotting?

In the color science community lots of people use matlab. I find matplotlib and numpy to be a great combination.

I've used VTK before. The learning curve is much larger and it's a huge library. But it does have a lot of inbuilt functionality like database access with filters/conversions to format the data the way you need it.

Physicist here.. I use CERN ROOT as it does just about everything. Occasionally I'll mix it up with python and matplotlib. GNUPlot is still great though too.

Tulip is great if you deal with huge datasets that can be represented as graphs.

http://tulip.labri.fr/

In engineering we use a lot of Matlab.