autocomplete/data assignment not working

I recently ran into problems assigning new names to data, i.e.:

gnuplot> data='080730-1333-avg-655-705.wl'
              ^
         invalid expression

this same syntax worked for me before and the data will plot. I'm
using gnuplot 4.0 on MacBook Pro OS X 10.4.11.
Unsure if it's related, but autocomplete for filenames isn't working
either.
Any suggestions?
cheers