Fix dot_graph
Fix dot_graph

Use subprocess.check_call so we get an error message if the command
fails (e.g., because graphviz is not installed).

Actually generate pngs (not pdfs) so visualization in notebooks works.

ssanderson/dask,cpcloud/dask,mrocklin/dask,jayhetee/dask,vikhyat/dask,pombredanne/dask,PhE/dask,mrocklin/dask,gameduell/dask,clarkfitzg/dask,simudream/dask,blaze/dask,mraspaud/dask,jcrist/dask,mikegraham/dask,dask/dask,dask/dask,jcrist/dask,chrisbarber/dask,cowlicks/dask,clarkfitzg/dask,ContinuumIO/dask,simudream/dask,jayhetee/dask,jakirkham/dask,ContinuumIO/dask,mraspaud/dask,pombredanne/dask,jakirkham/dask,PhE/dask,wiso/dask,blaze/dask,vikhyat/dask,ssanderson/dask,wiso/dask