One way is to display your image and then run BEAMFIT in cursor mode
with Parameter MARK set to "Ellipse".
display inti_g30p32+0p07 \\
beamfit inti_g30p32+0p07 cu mark=ellipse
If you want to input the co-ordinates of the beam in some other fashion,
then it's a little more work. BEAMFIT lets you save a map of the
residuals. A simple subtraction gives you the fit image (but BEAMFIT
could be made to offer the fit as an image instead).
Here's an example using your BEAMFIT invocation.
# Obtain an image of the fitted data.
beamfit inti_g30p32+0p07 i 1 \"30.313,0.067\" resid=beam_resid
sub inti_g30p32+0p07 beam_resid beam
# Find the half maximum using the fitted amplitude and base level.
set amp = `parget amp beamfit`
set base = `parget back`
set halfmax = `calc exp="0.5*$amp[1]+$base[1]"`
# Contour over the existing plot. You can select the plotting style
# too such as the colour or line width of the contour.
contour ndf=beam noclear noaxes mode=free heights=$halfmax
Malcolm
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