How to place labels on top of each box in gnuplot?

I have an array of data which looks like this [0.5, 0.3, 0.15,0.0, 开发者_运维技巧0.0, -0.09, -0.25,-0.4].

I wanted put a corresponding label (or number) of the boxes on top of the each box to indicate which one is which.

Anybody knows how to do this ??

Thanks.

plot 'data.txt' using 1:3 with boxes lt 1 title columnheader(3), \
     'data.txt' using 1:3:(sprintf("%.0f",$3)) with labels font "Helvetica,8" center offset 0,0.5 notitle