```vc - make a velocity-space contour map

EXAMPLES:
As with cm, the best plan is probably to start using vc ?! and answer the
questions.  Then start typing changes to the last vc map directly:

vc st:.5		changes the step size to 0.5
vc vl:-3,5.5		change the limits of velocity which are plotted to
-3 to 5.5 Km/s.
vc mlb:"XXXX"		to change the title to XXXX.

Eventually you should type in new vc maps directly:

vc 0,-12,2,10,1 ir:1.1 ci: vl:8,12 st:1	This will make a space-vel.
map along the line from (0,-12) to (2,10) with
1 'arc point spacing, cone interpolate with a
radius of 1.1', a velocity scale covering 8 to
12 Km/s and contours every 1K.

DESCRIPTION:
Vc makes space-velocity image along a line between two given points in
the current relative coordinate system.  The data will be interpolated
onto points spaced by 'sp' arcmin before plotting.  If the line is
along a line of observations sp should be picked so that the plotting
points are commensurate with the data points.  A contour map of
the image will be made and other maps and FITS files can be made
from image 1.

Vc needs an example stack in stack 1 to determine the number of
channels on the vertical (velocity) axis of the map.  It remembers the
filter width in that stack and only uses stacks with that filter
width.

Velocity limits set the set the velocity range of the plot.  Only the
channels within  that range are plotted.  If a stack does not cover the
full range of the plot, the missing points are given the value DRAGON
(IEEE NaN) and not plotted.

Interpolation radius and type have the same meaning as in cm.  If an
averaging type of interpolation is used, the appropriate stacks are
averaged before being contoured.

Stack number limits and center frequency limits can be used to exclude
some stacks from the map.

The contour level selection is the same as in cm.  Old contour values
and step sizes are stored separately from cm values.

If you supply labels, they override the default labels for the current
plot only.

'vcli' is a macro in .GMACROS which will make a spatial-velocity diagram
along the line between two cursor points.

```