

   VVaarriiooggrraamm CClloouudd

        spacecloud(point.obj, pairs.obj, v1, v2=v1, type='r', query.a)

   AArrgguummeennttss::

   point.obj: a point object generated by `point()'

   pairs.obj: a pairs object generated by `pairs()'

         v1: a variable to plot

         v2: an optional variable name, if entered the plot
             will be created between `v1' and `v2'

       type: either `'r'' for square-root differences or `'s''
             for squared differences

    query.a: an optional variable name, if entered, the value
             of the variable will be displayed on the graphics
             device for points identified by the user.

   DDeessccrriippttiioonn::

        Create a scatter plot of square-root or squared differ
        ences between variable values at pairs of points versus
        the distance between the points.

   VVaalluuee::

        NULL

   NNoottee::

        When `query.a' is entered, the user will be prompted to
        identify points on the display device. Because each
        point in the plot represents a pair of locations, the
        user must identify each point twice, once for the
        "from" point and once for the "to" point. Querying is
        ended by pressing the middle mouse button on the mouse
        while the cursor is in the display window.

   RReeffeerreenncceess::

        http://www.gis.iastate.edu/SGeoStat/homepage.html

   SSeeee AAllssoo::

        `point', `pairs'

   EExxaammpplleess::

        spacecloud(maas.pts,maas.pairs,'zinc')
        spacecloud(maas.pts,maas.pairs,'zinc',query.a='zinc')

