I know I'm late to respond, but depending on what type of thing you're prototyping, I would recommend Tableau.  When I prototype reports and dashboards, Tableau is ALWAYS my first stop (if not my last).  The UI is extremely accessible, plus it just puts out beautiful products.  You can check it out by downloading the public version of Tableau before you buy.

Barring that, in a previous position I just straight-up used C++ and SQL Server to create dashboards and reports that were headed for production but needed some fine-tuning.

Hopefully that's helpful in some way.