This is a useful function for debugging. It pops up a graph on the
computer screen (using the graphing program `xmgr`) showing a graph
of some array which you happen to want to look at.

The arguments are:

`array:`Input. The array that you want a graph of.`n:`Input. The number of array elements that you want to graph.`spacing:`Input. The spacing of the array elements that you want to graph. The elements graphed are`array[0]`,`array[spacing]`,`array[2*spacing]`,...,`array[(n-1)*spacing]`.

This function is a handy way to get a quick look at the contents of some
array. It writes the output to a temporary file and then starts up
`xmgr`, reading the input from the file. The values are evenly
spaced integers from to `n-1`. The values are the (subset of)
points in `array[ ]`. If your array contains real data, you might want
to use `spacing=1`. If your array contains complex data (with real
and imaginary parts interleaved) you will use `spacing=2`,
and make separate calls to see the real and imaginary parts. For
example if `complex[0..2047]` contains 1024 complex numbers, then:
`graph(complex,1024,2)` (view 1024 real values)
`graph(complex+1,1024,2)` (view 1024 imaginary values)

Note that in order not to produce too much garbage on the screen,
any output or error messages from `xmgr` are tossed into
`/dev/null`!

- Authors: Bruce Allen, ballen@dirac.phys.uwm.edu
- Comments: None.