plotstring function requires a configuration option for "using" statement

[LukasKrumwiede]

If one wants to define the border for filling the space between two curves it requires an option to pass the using argument in the plotstring function. I tried it with curveconf! but the returned string looks like
plot 'Temp\jl_VVb16f2sAr' i 0 title 'y1' using 1:2:3, but it should have the form:
plot 'Temp\jl_VVb16f2sAr' using 1:2:3 i 0 title 'y1' w filledcurves