On Aug 9, 2015, at 8:38 AM, Alan BRASLAU <alan.braslau@cea.fr> wrote:
> 
> One needs to think about closing the data file as ConTeXt opens a single
> MP instance that is run throughout the entire document AND over its
> multiple runs. If the file is left open, a second run will try to read
> on from the point where it left off.
> 
> THIS IS A SUBTLE POINT THAT SHOULD BE NOTICED BY ALL MPLIB USERS!

Thank you for this insight Alan! Two results immediately helpful are:
1) data can be put at the top of the .tex file
2) autogrids can be rendered and then drawn over

The following may not be the best way, but it works to read data from the top of the .tex file and then plot on the autogrid.

% 1 0.5 2.5
% 2 1 3.5

\usemodule[graph]
\startMPpage[instance=graph,offset=2mm]
  draw begingraph(3cm,5cm);
  numeric a[];
    gdata("read1.tex", v,
      setrange(0,0,3,4);
    );
    closefrom "read1.tex";

    autogrid(,grid.lft) withcolor .7white;

    gdata("read1.tex",v,
    for j = 1 upto 3:
        a[j] := scantokens v[j];
      endfor;
      path b;
      augment.b(a1,a2);
      augment.b(a1,a3);
      setrange(0,0,3,4);
      gdraw b;
    );
    closefrom "read1.tex";
  endgraph;
\stopMPpage