Well it wasn't pretty, but it was done. Leveraging as much code as possible from Before Tax I made a bare bones widget: a form with a two-column table. Technically speaking it was a big fat blob of HTML, but it was more or less what the team at Amy Northard CPA needed for their project.
I was using this whole experience to glean as much user feedback as possible. To my surprise all the "critical" features in my backlog were an afterthought. Users wanted pragmatic, unsexy, widget UI's and easier installation.
What matters most at this stage is simplicity. No fancy charts, no endless styling options, just presenting the data right on user's sites. After talking with other prospects, another key concern was ensuring they don't lose hours struggling to publish their widgets.
So in effect the biggest concerns where surprisingly... boring: present spreadsheet results simply and make installation fast/easy. Everything else is a bonus.