better control of balloon positioning

[Werner LEMBERG]

A balloon's `X-offset` property results in the following three
alignments for a given x value.

  x < 0:  Balloon text
                      \
                       \
                        grob

  x = 0:             Balloon text
                          |
                          |
                        grob

  x > 0:                      Balloon text
                             /
                            /
                        grob

This shows that `X-offset` controls the attach point alignment on both
sides of the annotation line, which is certainly a good default.
However, it isn't sufficient in crowded situations.  What must I do to
get, for example,

                  Balloon text
                             /
                            /
                        grob

this alignment?  Please advise.


    Werner