The sixth graffiti is in the hospital and it is maybe slightly glitched at the moment. Normally turn left at the beginning of the hospital and you see a vending machine. Interact / slam the vending machine 3 times and the option for the graffiti should be appears. (atm not until patch?)
Currently you have to obtain as first graffiti #7 then you get the option for the graffiti #6 on the vending machine. So go right in the hospital to trigger the cutscene with the couple from Episode 2. After this cutscene interact with a nurse poster on the cork board (graffiti #7) and go back to the vending machine and you will have the option for the graffiti #6
For a visual guide open the spoiler, inside you find my video with all 10 graffiti locations.