In this video Sachin Patney and Nico Cristache describe the process of creating custom visualizations for Power BI. It is remarkable how easy it is to create any kind of visualization given the framework that's been set up. The best part of their explanation is where they detail the fact that everything is open source! In essence when you create a custom visualization, you're not playing in some sandboxed area, you're literally creating the same kind of visualizations that already exist natively in the system. There's also a surprise contest announcement at the end!
Some useful links: