I've considered two other ways to approach this:
1) Create an array of DataReferenceImpl's that do pretty much the
same thing (redraw the radial when moved - like the AnchoredPoint
method) and space them so that the user should (almost) always have
one visible no matter how they manipulate (pan/zoom) the display.
2) Somehow change the endpoint location of the DataReferenceImpl to
always be in the viewable area. The difficult part of this is to move
the DataReferenceImpl down the existing radial (along and toward the
Another option is to do manual picking. It takes more code than a direct
manipulation renderer, but you have more control. (And in your case, it
may be less code than dealing with N direct manipulation renderers
distributed along the line.) I have posted an example called
RadialLine.java on my VisAD examples page at
http://www.ssec.wisc.edu/~curtis/examples/ that demonstrates this. It
allows you to pick anywhere along the radial line using the right mouse
button as you describe.