plot \
'df001/doccmp001.pic' using 3:5 title 'I' with lines,\
'df002/doccmp001.pic' using 3:5 title 'II' with lines,\
'df003/doccmp001.pic' using 3:5 title 'III' with lines,\
'df004/doccmp001.pic' using 3:5 title 'IV' with lines,\
'combires.pic' using 3:5 title 'Combined' with lines