x, y
Return the value of the map pixel nearest to a given coordinate.
EXAMPLE
-------

1. The following use of mapval() displays the flux in the map at
   a position 3.5 arcsec to the east and 0.7 arcsec to the north
   of the center of the map.

    0>mapunits arcsec
    Distances in the map plane now have units: arcsec.
    Distances in the UV plane now have units: kilo-wavelengths.
    0>print mapval(3.5,0.7)
    0.293557
    0>

2. The flux at the center of the map can be displayed with:

    0>print mapval(0,0)
    3.44326 
    0>

PARAMETERS
----------

x     - The X offset with respect to the center of the map, as
        displayed by maplot.

y     - The Y offset with respect to the center of the map, as
        displayed by maplot.

SIDE-EFFECTS
------------
If the map is out of date with the UV data then it will be re-inverted
before returning a value.

RELATED FUNCTIONS
-----------------
peak    - Return details of the min,max or absolute max flux in a map.
imstat - Return image plane map statistics.