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While the derivative classification rule applies on the originating agency side of things (where agencies must appropriately mark documents with the highest level of classification of the document equal to that of the document from which the classified information came), there is no parallel requirement on the researcher side.
As you note, CFR 1254.76 requires that copies of declassified documents can only leave NARA research spaces with a declassification marking, which is understood to be the declassification project number under which the records were declassified. These markings usually bear the letters NND or NW followed by a five or six digit number. I recommend that a footnote for the graphic along the lines of "Information on this graphic derived from declassified sources at the National Archives at College Park (or other specific NARA facility) from declassification project NND XXXXX" would be helpful in the situation you describe.