nt color="#82140F"><code>read columns x y</code></font>' tells Gri to start reading columnar data,
the first column being `<font color="#82140F"><code>x</code></font>', the second `<font color="#82140F"><code>y</code></font>'.  `<font color="#82140F"><code>x</code></font>' and
`<font color="#82140F"><code>y</code></font>' are predefined names for whatever ends up on the horizontal and
vertical axes.  
<p>

The number of data needn't be specified.  Gri reads columns until a
blank line or end-of-file is found