Yeah, this is a bug, I reported it to Coda a few days ago.
In my dashboard sample, the control for pie chart affects the column values, not the filter. I think there may be some workaround but I haven’t looked into that yet.
UPD: The workaround would be to add a column to your sprint’s table that would simply pull the value from the select from your dashboard (e.g. same value for all rows if all you need is just a count), then make your chart dependent on that value. Then it will re-render when the select is changed:
Updated the sample in the original post with this workaround.