add prim-9
brush the first 4 points in order to draw the brush rectangle
extend the introduction a little bit and add references to gapminder, manyeyes and d3
add the plumbing references
I do not know why these changes were reverted in 37f86064a554db8e50d0b1ae4beacbc3689a2051
update the caption of Figure 1
explain the "brush"