graph.js: redo the svg height setting

I hope this is easier to read and better matched what @domoritz
requested in
https://github.com/uwescience/myria-web/pull/197#issuecomment-54386528.
Or at least is better explained.
editor: switch the panel collapse settings

By default, make the logical plan collapsed and the physical plan shown.
First, this is usually correct. Second, this fixes a bug where the size
of the SVG was not being accurately updated because the panel was
closed.