Adding the data is by far the easiest to do. There is really no reason not to do it like that. it works well even if you have 100 tags and 100set points. I usually use a normal graph or chart, not a wafeform, but that should not make much difference.
You might think it's a waste of memory,to add the exact same value, because the set point does not change. The best next solution is to use an XY graph. But now you need to add a timestamp to each sample of the first set of data, and the st pont samles. You end up using even more memory. This is only uefull if you'd have one data set, and severl sets taht change less often.
Another radical solution is to use the plots front image. You can draw anything in it, including setpoint lines. It's usfull for advanced anotations and custom data visualisation, but in this case there really is no point. See this presentation.
Regard,
Wiebe.