Graffiti, which also serves as the plural of graffito, is commonly used as a singular mass noun <graffiti … was depressing people who rode the subways — New Yorker><graffiti comes in various styles — S. K. Oberbeck>. This use is well established although not yet as well established as the mass-noun use of data. Use of graffiti as a singular count noun is still quite rare and is not standard.
Examples of GRAFFITI
The walls of the old building are covered with graffiti.