I had the same idea and put an optional field in the properties file as well,
and now that I think about it, the way you have suggested is much cleaner. When
I added the help for the csv file, it made the help very long winded and confusing.
You can see my changes here,
I did some refactoring inside to support csv, which ended up in iterating over a
set of plot points, this may be useful for reading XML as well.
Look at PlotPoint.java and PlotPointFactory.java in my patch, I think that we
can use this as a way to abstract out where the plot data comes from, and with a
little work, we can setup some abstraction to use the value set in the UI to
create a PlotPointFactory that will generate PlotPoints. It looks like you had
some of the same issues I had with extra data.
One of my needs, which it seems you may have it is to have multiple points from
the same file.
If you want to rework your patch with the suggestions below I'll rework mine and
get Nigel to apply them.
Unit tests would be great, I was looking for some when I made my changes, but
didn't find an easy way to add them.