Thank you Alan. setrange is the key I had not tried and I will be more careful with (2).

This may not be the proper way to ask, but it is related to the topic: how to get Hobby’s Figure 9 from the mpgraph manual to work using m-graph?

http://ctan.org/tex-archive/systems/doc/metapost/source-manual <http://ctan.org/tex-archive/systems/doc/metapost/source-manual>

Thanks again,
John


On Aug 7, 2015, at 6:23 AM, Alan BRASLAU <alan.braslau@cea.fr> wrote:
> 
> You have two problems:
> 
> 1) gdraw (4,1)--(7,1);
> has a unique ordinate, so graph will fail in this case if you do not
> explicitly set the range.
> 
> 2) You have to take care with your choice of loop variables in MP not
> to run into conflicts those eventually used in macros. Also, as Hans
> indicated, you do need to declare that a is an array, as in
> 	numeric a[];
> 
> Here is a MWE
> 
> \enabletrackers[metapost.showlog]
> \usemodule[graph]
> \startMPpage[instance=graph,offset=3mm]
>  draw begingraph(5cm,5cm);
>    numeric a[];
>    gdata("data.d", v,    
>      for j=1 upto 3:
>        a[j]:=scantokens v[j];
>      endfor ;
>      );
> 
>    setrange (whatever,0,whatever,2);
>    gdraw (a2,a1)--(a3,a1);
>  endgraph;
> \stopMPpage
> 
> 
> Alan
> 
> 
> On Thu, 6 Aug 2015 16:48:51 -0400
> John Kitzmiller <kitz@inradius.net <mailto:kitz@inradius.net>> wrote:
> 
>> On Aug 6, 2015, at 12:59 PM, Hans Hagen <pragma@wxs.nl <mailto:pragma@wxs.nl>
>> <mailto:pragma@wxs.nl <mailto:pragma@wxs.nl>>> wrote:
>>> 
>>> 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.
>> 
>>> 
>>> also, use
>>> 
>>> a[n] := scantokens v[n] ;
>> 
>> Done. Still the same error messages and no pdf.
>> 
>>> 
>>> 
>>> 
>>>> My goal is to draw multiple box plots
>>>> https://en.wikipedia.org/wiki/Box_plot <https://en.wikipedia.org/wiki/Box_plot>
>>>> <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.
>> 
>