#plot of a function bounded by other functions in the plane 
f(x,y) = x**2 + y**2
x(u) = 3*u
yu(x) =  x**2
yl(x) = -x**2
set parametric
set cont
splot [0:1] [0:1] u,yl(x(u))+(yu(x(u)) - yl(x(u)))*v,\
f(x(u), (yu(x(u)) - yl(x(u)))*v)