non-linear curve-fitting (for 3 parameters):
y = a0*x^2+a1*x + a2

y = S0.y - S1.y
xmgrace -block file -bxyz 2:3

greek: use \x letter \f{}
super: use \S letters \N
sub:   use \s letters \N
Angstrom: use \cE\C
new line: \n

Example:
 F\sX\N(\xe\f{}) = sin(\xe\f{})\#{b7}e\S-X\N\#{b7}cos(\xe\f{})