ot again: gnuplot> how to get decimal hour exported

[Wolfgang Engelmann]

I run this:

gnuplot> plot "file.txt" using 0:($1 + $2/60.)
with data in file.txt like

   1	00 01
   2	00 04
   3	00 07
   4	00 11
   ...... up to 256

the minutes (3rd column) are divided by 60 and added to the hour (2nd 
column). They are plotted against time (1st column) and I get a correct 
graph.

How can I obtain the column of data with the hours and decimalized 
minutes, e.g. 00 30 > 00.50 ? I need them for further analyses.

I could not find the command in the gnuplot (and pyxplot) manuals. 
Perhaps I missed the search word for it.

I would appreciate very much help

Wolfgang