This is a two dimensional quadratic of the form:
f (u, v) = a0 + a1u + a2v + a3u2 + a4v2 + a5uv.
u and v are used instead of x and y or x0 and x1, because
gnuplot needs to plot the data parametrically. The linear
fitting algorithm works pretty well, but I recommend the
result be checked with a few iterations of the L-M
algorithm, for example, issue the command, fi 5 after the
li command.