Here is an API that draws UML diagrams exactly as Google Chart API draws charts : yUML !
An example to see it in action :
This link
http://yuml.me/diagram/scruffy/class/[note: Agafix Geek Process{bg:cornsilk}], [Internet]<-digs[Reddit], [Internet]<-reads[<>], [Taoufix]eats-.->[Reddit], [Taoufix]drunks-.->[<>], [<>]^-.-share[Taoufix], [<>]share^-.-[Taoufix], [<>]^-LOL[Fahdos], [<>]^-comments[Faycal], [<>]<-.-needs[Fahdos], [<>]<-.-needs[Faycal], [Taoufix]watches->[Agafix.org{bg:green}], [Fahdos]comments->[Agafix.org{bg:green}], [Faycal]writes->[Agafix.org{bg:green}]
draws this image
More examples at http://yuml.me/diagram/class/samples
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January 23rd, 2010
at 21:32
Other similar tools are listed here: http://modeling-languages.com/content/uml-tools#textual