If you want to run the graph iff the output file exists then you canwrite a script in start script to do the same.

psuedo script:

if out.dat not exists

exit

endif

Here is a simple example to use a start script in a graph:

In start script lets give as:

export $DT `date '+ m d y'`

Now this variable DT will have today's date before the graph is run.

Now somewhere in the graph transform we can use this variable as;

out.process_dt::$DT;

which provides the value from the shell.

Hope this helps...