The folks at Trifacta are making it easy to build visualizations on top of D3 with Vega. Vincent makes it easy to build Vega with Python.
The data capabilities of Python. The visualization capabilities of JavaScript.
Vincent takes Python data structures (tuples, lists, dicts, and Pandas DataFrames) and translates them into Vega visualization grammar. It allows for quick iteration of visualization designs via simple addition and subtraction of grammar elements, and outputs the final visualization to JSON.