Interactive Geometric Distortion Fitting
?	Print options
f	Fit the data and graph with the current graph type (g, x, r, y, s)
g	Graph the data and the current fit
x,r	Graph the x(in) fit residuals versus x(ref) and y(ref) respectively
y,s	Graph the y(in) fit residuals versus x(ref) and y(ref) respectively
d,u	Delete or undelete the data point nearest the cursor
o	Overplot the next graph
c	Toggle the constant x(ref), y(ref) plotting option
t       Plot a line of constant x(ref), y(ref) through nearest data point	
l	Print xshift, yshift, xscale, yscale, xrotate, yrotate
q	Exit the interactive curve fitting
v	Vector residuals plotted with chosen magnification

The parameters are listed or set with the following commands which may be
abbreviated.  To list the value of a parameter type the command alone.
:show				List parameters
:function [value]	        Fitting function (chebyshev, legendre)
:xxorder :xyorder [value]	X fitting function xorder, yorder
:yxorder :yyorder [value]	Y fitting function xorder, yorder
:xxterms :yxterms [y/n]		Include cross-terms in X, Y fit
:reject [value]			Rejection threshold
:magnification [value]		Magnification factor for vector residuals