Knowing other languages is great, but there is very real overhead in learning them. Python is mature enough that you have mature libraries available for pretty much everything.
and frankly, Python has no libraries for interactive visualization in your browser because only JavaScript runs there.
etc.
"Bokeh is a Python interactive visualization library that targets modern web browsers for presentation."
(Of course, it works by generating JavaScript)
http://bokeh.pydata.org/en/latest/docs/user_guide/interactio... see the CustomJS function.
Isn't that like saying that C has no visualization libraries on Intel processors because only assembly runs there?