  With the growing popularity of Non-Fungible Tokens (NFT), a new type of
digital assets, various fraudulent activities have appeared in NFT markets.
Among them, wash trading has become one of the most common frauds in NFT
markets, which attempts to mislead investors by creating fake trading volumes.
Due to the sophisticated patterns of wash trading, only a subset of them can be
detected by automatic algorithms, and manual inspection is usually required. We
propose NFTDisk, a novel visualization for investors to identify wash trading
activities in NFT markets, where two linked visualization modules are
presented: a radial visualization module with a disk metaphor to overview NFT
transactions and a flow-based visualization module to reveal detailed NFT flows
at multiple levels. We conduct two case studies and an in-depth user interview
with 14 NFT investors to evaluate NFTDisk. The results demonstrate its
effectiveness in exploring wash trading activities in NFT markets.
