function ip_fit(xa,ya,xi,yi)
% Interactively fit instrumental profiles to arc data
% Function to interactively fit instrumental profiles
% to arc data.  Note that IP is scaled and shifted in
% both X and Y to fit data.
% To use: ip_fit(xa,ya,xi,yi)
%
% by D. Holmgren, Feb 16 95 holmgren@phobos.astro.uwo.ca
xz = input('   X zero-pt.: ');
xs = input('X scale fact.: ');
yz = input('   Y zero-pt.: ');
ys = input('Y scale fact.: ');
plot(xa,ya,'+',xz + xi./xs,yz + yi.*ys)