added url option for Tree plots

[mrjoh3]

Hi, I have added a small bit of code so that elements in a tree map can be used as links to other sites. I have tried to keep the addition as small as possible to maintain current functionality.

A URL can now be specified in column four of the data input. Using the column position means that to use URL's the color column also needs to be specified.

When the URL column is present all other mouse events and the legend are disabled. A future update could allow for these settings to be modified.