Rectangular zones in graphs

Hi there,

I want to highlight a certain "corridor" in a graph. In other words, a
zone in the range [y1:y2] should have a different colored background or
be overlayed by a semi-transparent box. I wonder if that is possible at
all with the Lout @Graph and @Diag calls. The user manual states it is
possible to place any arbitrary object in a grap, however, I have a hard
time figuring out how this should work. I even fail to draw a box inside
the graph, see this try expanding the example of the manual. My task is to
mark the band between 4 and 5 on the y-axis, but using @Node outline and
@Link fails with `undefined error: unknown or misspelt label? Command:
A` when piping the lout output to ps2pdf. Is this approach correct at
all or do I have to use @Box, other coordinates … ?

  @SysInclude { graph }
  @SysInclude { diag }
  @SysInclude { doc }
  @Doc @Text @Begin
  @Diag {
  @Graph
  abovecaption { New South Wales road deaths, 1960--1990
  (fatalities per 100 million vehicle km) }
  objects {
   @CTR at { 1960 4 } { A:: @Node outline {} }
   @CTR at { 1990 4 } { B:: @Node outline {} }
   @CTR at { 1990 5 } { C:: @Node outline {} }
   @CTR at { 1960 5 } { D:: @Node outline {} }
  }
  {
  @Data points { plus } pairs { dashed }
  { 1963 5.6 1971 4.3 1976 3.7 1979 3.4 1982 2.9 1985 2.3 1988 2.0 }
  }
  @Link from { A } to { B }
  @Link from { B } to { C }
  @Link from { C } to { D }
  @Link from { D } to { A }
  }
  @End @Text

Any help is welcome, thanks for it in advance!
−Martin