Link Behavior

OK, so the answer to this question may be that I am barking up the wrong tree and that the link behaviour is not the way to go....

In the image below I want to be able to move "a" and I want the line linking it to move with it (à la mind map.) The link behaviour will simply move the entire object (the line in this case) whereas I am looking for a solution (which may not exist) that would bend and stretch that line. Where only one anchor point of the line would be linked to "a"

This is more for workflow purposes than anything else, as I have a bunch of mind maps to animate...

Thanks Simon,

That very helpfully put me on the right track and I see now that I very rudely failed to thank you.

Thank you,

T