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…


My goal is to draw multiple box plots https://en.wikipedia.org/wiki/Box_plot <https://en.wikipedia.org/wiki/Box_plot>, either vertical or horizontal, from data sets, but I am stuck at this early stage.