control tree explorer with graph.
the selected node is automatically expanded and scroll to in the tree
even if it is hidden.
it finally rotates. However only clock wise.
Maybe I should use tan instead of cosine.

Long time debug!
1. sqrt the lengh!
2. what I got is cos theta, NOT theta it self!

3. my high school math is really bad, I don't know the range theta get
from arcosine.
Rotation is pretty good now.
But it's a whole ratation.

Next thing to do is to only rotate the desendants.