Thank you to everyone who offered help with this question. I now see it was far more complex than I had thought :) Not surprising really. If I get any figures from some of the methods suggested I will post them back to the list. Thanks! jon UTS CRICOS Provider Code: 00099F DISCLAIMER ======================================================================== This email message and any accompanying attachments may contain confidential information. If you are not the intended recipient, do not read, use, disseminate, distribute or copy this message or attachments. If you have received this message in error, please notify the sender immediately and delete this message. Any views expressed in this message are those of the individual sender, except where the sender expressly, and with authority, states them to be the views the University of Technology Sydney. Before opening any attachments, please check them for viruses and defects. ========================================================================