Gnuplot: splot with data-file only containing x and b(x)

[flyartemis]

I want to plot the contour of the following function:

f(x,y)=y³*b(x)

My data-file looks something like this:

x b(x)
-1 10.123
-0.995 10.112
-0.99 10.100

I am not sure how to make the right splot command or how to solve this problem at all as my datafile does not look like (x y z).

Thats my script so far:

reset
f(x,y)=y³*b(x)
set xrange [-6:6] 
set yrange [-5:5]
set isosamples 50
set table 'test.dat'
splot 'Data.dat' u 1:(b(x)=$2, f(x,y))     -------------------------?
unset table

set contour base
set cntrparam bspline
set cntrparam levels incremental -0.1,0.02,0.1
unset surface
set table 'contour.dat'
 splot 'Data.dat' u 1:(b(x)=$2, f(x,y))     -------------------------?
unset table

reset 
set xrange [-6:6]
set yrange [-5:5]
unset key
set palette rgbformulae 33,13,10
plot 'test.dat' with image, 'contour.dat' w l lt -1 lw 1.5