This issue was originally opened for sprint support in the Classic Cumulative Flow Diagram gadget. It has since become a mixture of that request and the need for the same gadget for the (newer) Agile board report.
So I will change the original summary to reflect this state.

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Original summary
Wallboard Cumulative Flow Chart Gadget cannot work with Rapid Board Sprints

Original description

This comes from a dedicated customer using the Cumulative Flow Chart Gadget with the Classic Planning Board.

Reproduction steps:
# You create a new Rapid Board, named RP1 (e.g. a Scrum one). In RP1 you include two projects, Pr1 and Pr2.
# Then you create a Sprint, named S1
# In RP1, you drag and drop issues from both Pr1 and Pr2 to S1.
# Unlike the classic Planning Board (where issues are actually transferred into versions of the actual one project they are related to), issues are only transferred into S1. This is visible on the bottom right part of the issue in JIRA (under the Agile section)
# In RP1, you can click on Report and then, instead of Sprint Report, select Cumulative Flow Diagram. This depicts the Cumulative Flow Diagram for S1 (and can act as a temporary workaround) but:
# This is something you cannot have in the GreenHopper Cumulative Flow Chart Dashboard Gadget (also Wallboard capable) as you can only select a version, not a sprint.

Question: Shouldn't the Cumulative Flow Chart Gadget be able to work with Sprints instead of versions only?