this is how a city supports graf :
DC (despite a long and storied history of important graf culture) has a group of gentrifiers that make their presence known at various local blogs about their hatred of graffiti. Not often do readers get so upset about a topic as much as this one.
SF (who also has a more relevant contemporary arts community, just FYI) often takes a different approach. Like the huge tag (not a mural, but a tag) by SF legend AMAZE that went up before the Haight-Ashbury Street Fair. It was commissioned by the buildings owners and covers the old Gap location. Some more at SF MetBlogs.
(Fun fact: the Gap started in the Haight as a blue jeans and record store, they thought about calling it PAD -- Pants And Discs.)