Other sources
Source: Banging Your Head Against a Brick Wall
Context: Bus stops are far more interesting and useful places to have art than in museums. Graffiti has more chance of meaning something or changing stuff than anything indoors. Graffiti has been used to start revolutions, stop wars, and generally is the voice of people who aren't listened to. Graffiti is one of those few tools you have if you have almost nothing. And even if you don't come up with a picture to cure world poverty you can make somebody smile while they're having a piss.
Quotes about graffiti
A collection of quotes on the topic of graffiti, doing, people, likeness.
Quotes about graffiti
“People say graffiti is ugly, irresponsible and childish. But that's only if it's done properly.”
Source: Wall and Piece (2005)
Source: Magic Breaks

“Graffiti is beautiful; like a brick in the face of a cop.”

“Poetry is eternal graffiti written in the heart of everyone.”
Source: Americus, Book I
Source: The Romantic Rebellion (1973), Ch. 4: Ingres I: The Years of Inspiration

“I am writing graffiti on your body.
I am drawing the story of how hard we tried.”
Both Hands
Song lyrics

On Wikipedia
Canadian pundit, Wikipedia founder in messy breakup http://web.archive.org/web/20080304021035/http://www.cbc.ca/canada/british-columbia/story/2008/03/02/marsden-breakup.html
Ron Finley at TED2013 (2013)
Source: The Coyote Kings of the Space-Age Bachelor Pad (2004), Chapter 34 “On Good, Evil, Invisible Hands, and the Wind” (p. 192)

talking about Guerilla Communication strategies in "Urban Hacking" http://www.transcript-verlag.de/ts1536/ts1536.php, transkript, p. 106
(Tristan Manco. Stencil Graffiti)
Other sources
Wall and Piece (2005)

All available evidence, however, points to the contrary.
Bongo in Childhood Is Hell (1988)
Source: Beyond the Chocolate War (1985), p. 40
Reconciliation, Tolerance, and Unity Bill, 24 July, 2005

"The Pleasure of the Image" (1985) from Writers on Artists edited by Daniel Halpern (1988), p. 98, North Point Press ISBN 0-86547-340-4
Pope Leo XIV in Ch. XV
Lazarus (1990)
These disciplines of inverse ascetism, one sees, mean shooting smack until you drop dead.
Page 195
Culture of Complaint (1993)
Dick Hebdidge (1979). . p.106-12

Tàpies is referring to the Franco-repression in Spain.
1945 - 1970, A Report on the Wall' 1970
Existencilism (2002)
Wall and Piece (2007)