function grav_plot(r,z)
clf;
input = load('gravity.dat');
x  = input(:,1);
dg = input(:,2);
n  = length(x) ;
k = 2 * pi * 6.67e-11;
drho = -2500 ;
plot(x,dg,'b-');
hold on
for i = 1:n
   dgp(i) = k * drho * r^2 * z/(x(i)^2 + z^2);
end;
plot(x,dgp,'r-');
axis([min(x),max(x),-0.000002,0.000001]);
xlabel('X distance [m]');
ylabel('measured gravity change');