I've now had more time to put DataGraph to use and it is a truly amazing program. Aside from it's performance, which is incredible, I keep finding new things I can do with it. For example, I can generate plots by using one .dgraph file as a template and feed new sets of data into it from the command line (or from a script), and output a .pdf of the new graph, or whatever format. Plus I can set the value of variables, such as the title of the graph, from command line or script.
There are some bugs, for example I couldn't get my graph-title variable to go into the Title field. Selecting it from the pop-up menu did nothing. However, I found that I could put it into a different field and then drag it to the Title field, and that worked, so obviously this is just a bug that will hopefully get fixed. In any case I've got a workaround for now.
So, if you need to plot data on the Mac, this is a fantastic program to choose, and it's well worth the asking price. Highly recommended!