Python or Tales expression in Graph Point Display

I am doing something similar to this:

http://wiki.zenoss.org/Per-Filesystem_Thresholds

which works well, but I would also like to be able to display the value I am producing within the graph point display. It seems that I can display cProperties on there own using

${here.cProperty)

What I do not seem to have working is the ability to display what I would think could be accessible with the same syntax, but as call to the python script which would have parenthesis:

${here.getFileSystemThreshold()}

Am I missing something Is this possible