Improve rendering of DOT diagrams.
Improve rendering of DOT diagrams.

Using a square instead of a rectangle made for inconvenient layouts
when there are complex graphs. Using a rectangle continues to provide
visual uniqueness between collections and transforms while allowing for
more compact pictures.

apache/beam,markflyhigh/incubator-beam,charlesccychen/beam,chamikaramj/beam,tgroh/beam,rangadi/incubator-beam,robertwb/incubator-beam,charlesccychen/incubator-beam,markflyhigh/incubator-beam,lukecwik/incubator-beam,iemejia/incubator-beam,robertwb/incubator-beam,charlesccychen/beam,rangadi/beam,markflyhigh/incubator-beam,apache/beam,chamikaramj/beam,rangadi/beam,tgroh/beam,charlesccychen/beam,rangadi/beam,RyanSkraba/beam,RyanSkraba/beam,chamikaramj/beam,rangadi/incubator-beam,apache/beam,tgroh/beam,mxm/incubator-beam,robertwb/incubator-beam,charlesccychen/beam,rangadi/beam,iemejia/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,rangadi/beam,robertwb/incubator-beam,markflyhigh/incubator-beam,apache/beam,markflyhigh/incubator-beam,lukecwik/incubator-beam,RyanSkraba/beam,tgroh/beam,RyanSkraba/beam,chamikaramj/beam,chamikaramj/beam,rangadi/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,charlesccychen/beam,charlesccychen/beam,tgroh/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,RyanSkraba/beam,charlesccychen/incubator-beam,RyanSkraba/beam,apache/beam,apache/beam,markflyhigh/incubator-beam,markflyhigh/incubator-beam,robertwb/incubator-beam,charlesccychen/incubator-beam,mxm/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,rangadi/beam,robertwb/incubator-beam,robertwb/incubator-beam,rangadi/beam,RyanSkraba/beam,lukecwik/incubator-beam,charlesccychen/beam,apache/beam,tgroh/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam