If I read your post correctly, you have two tables (graffiti and garbage bags collected), and you want data from both tables in the same report even though there is no relationship linking the two lists of records that you want.
This can be done, but you have to get creative. Essentially, you have to come up with an approach that loads all your records from two tables into a unified "report table". Now that the report data is stored in or referenced by a single table, you can get your report printed out. To group your records, you label each record as to whether it is a graffiti or garbage bag record and sort on that label field. You can also use Sub-Summary layout parts to label each field.
This is only a general outline. The challenge lies in implementing the details.