Syntax: Use values from dataset as reference line in scatterplot

#1
I have a dataset with K subjects, all analyzed for L aspects, M parameters per aspect and the mean and difference of 1 of these M parameters. After Layered by ASPECTS, using

GGRAPH
/GRAPHDATASET NAME="graphdataset" VARIABLES=read write
/GRAPHSPEC SOURCE=INLINE.
BEGIN GPL
SOURCE: s=userSource(id("graphdataset"))
DATA: read=col(source(s), name("read"))
DATA: write=col(source(s), name("write"))
GUIDE: form.line(position(*,60), color(color.red))
ELEMENT: point(position(read*write))
END GPL.
[taken from http://www.ats.ucla.edu/stat/spss/library/ggraph_examples.htm] I am able to create scatterplots with reference lines manually, but I would like to use a variable value in the dataset to replace the "60" or "65" in the code above. How can I do this?

I tried

GGRAPH
/GRAPHDATASET NAME="graphdataset" VARIABLES=read write ref
/GRAPHSPEC SOURCE=INLINE.
BEGIN GPL
SOURCE: s=userSource(id("graphdataset"))
DATA: read=col(source(s), name("read"))
DATA: write=col(source(s), name("write"))
DATA: ref=col(source(s), name("ref"))
GUIDE: form.line(position(*,"ref"), color(color.red))
ELEMENT: point(position(read*write))
END GPL.
Added is ref (constant value per ASPECT, present for each SUBJECT) in line 2, in DATA and in GUIDE. This does not work, however.:(