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
New Yorker          
  
        
      
          
          
      
      
         
                        graffiti comes in various styles      
    
        
        
            — S. K. Oberbeck
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.
            
              graffitied; graffitiing
                                        grə-ˈfē-(ˌ)tē-iŋ  
                                     
                                    
    gra-,
                                    grä-
    
 also graffiting
                                        grə-ˈfē-tiŋ gra-,
                                    grä-
    
            
        
                                
              
          
                                                      : to draw graffiti on : to deface with graffiti                                      
              
                             
graffitied walls
                         
                
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  Merriam-Webster unabridged




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