Possible Clues:

• Hardy's Diggory
• -- diagram (math image)
• -- diagram (logic tool)
• ___ diagram (infographic with overlapping circles)
• John who was a pioneer in set theory
• Logician known for overlapping diagrams
• Diagram variety with overlap
• Type of diagram with overlapping circles
• — diagram (logic image)
• Logician John, for whom overlapping-circle diagrams are named