Currently various gadgets (Pie Chart gadget, Two Dimensional Filter Statistics, Workload Pie Chart, Issue Statistics, Heat Map) all have a dropdown that lets you choose what statistic you are interested in. However this dropdown shows all fields, not just fields relevant to the filter/project. If you use Team managed projects all custom fields from those projects appear in the dropdown.

There should be a dependency between the filter/project and the statistics dropdown so you can only see fields relevant to the filter/project in question.