On 8/6/2015 5:30 PM, John Kitzmiller wrote:
Given a file, data.d, containing 1 4 7

I expected the following code to draw a line from (4,1) to (7,1)…

\enabletrackers[metapost.showlog]
\usemodule[m-graph]
\startMPpage[instance=graph,offset=3mm]
  draw begingraph(5cm,5cm);
    gdata("data.d", v,
      for n = 1 upto 3:
a[n] = scantokens v[n];
      endfor;

      show a1; show a2; show a3; % numeric
      show v1; show v2; show v3; % string
      gdraw (a2,a1)—(a3,a1); % same problem in or out of data group
      );
  endgraph;
\stopMPpage

…the show lines return what I expected, but then come the complaints:

! Inconsistent equation (off by 0.90000000000000002)…
(0,0,1,0,0,yypart S)
! Transform components aren't all known…

a[n] = scantokens v[n];

is a defined?

Thank you Hans. Do you mean numeric a[n]? It was not explicitly declared; I did not think it needed to be—the show commands write what I expect to the log—anyway, declaring a did not help.