Graffiti vs. Graphic Design

I've always had a question as to whether or not Graffiti is/could be considered a form of graphic design.  Graffiti, as we all know, is an art form who's main focus is the creation of graphical elements through the use of spray paint.  The origins of Graffiti go way back in history before people could give it a name.  One of the earliest forms is a caricature of a Pompeiian politician in the Ancient times.  

Typographical elements are a key part to Graffiti work.  Although considered to be illegal, because its defective nature, Graffiti often combines similar elements that exist in the world of Graphic Design.  Many of the issues that a typographer would have to address are the same ones that a tag artist would have to consider.  

The only outstanding difference between Graffiti art and Graphic Design is the readability of the typographical elements.  The main focus in Typography is to create a composition where the words are readable, legible, memorable, and scaleable.  I tag artist has some leg room when it comes to those issues.  

So, what do you think?  Could Graffiti be considered a form of Graphic Design?