tree c7889c5065ea919b2cc1da533c1b5a67c443c80d
parent b67053ce73ca62c9e543b9e9a0232fbfd9593b3f
author Avery Pennarun <apenwarr@gmail.com> 1353902734 -0500
committer Avery Pennarun <apenwarr@gmail.com> 1355123014 -0500

Add a 'tree' chart type and a 'treegroup' function.

These allow you to automatically create treemap chart hierarchies using the
levels of your key.  So treegroup=a,b,c gives you a tree with the toplevel
showing all 'a', then a second level showing all 'b' for each 'a', and so
on.