Wouter,
It sounds like you may want to try using the Symbol Routine using the "+" symbol instead. Here is an example:
p = PLOT(/TEST)
s = SYMBOL(50, 0, '+', SYM_COLOR='Blue', SYM_SIZE=2, $
SYM_THICK=3, /DATA, LABEL_STRING='My star')
s2 = SYMBOL(100, 0, '+', SYM_COLOR='Red', SYM_SIZE=2, $
SYM_THICK=3, /DATA, LABEL_STRING='My star')
For more information see this link:
http://www.exelisvis.com/docs/symbol.html
Best Regards,
-Lisa