font shown to be Courier Truetype (Macintosh OSX).

And why is the fount Courier instead of the expected ComputerModern  
SansSerif?

Contents of test.mpx is:

% Written by DVItoMP, Version 0.9/color (Web2C 7.5.3)
begingroup save _p,_r,_s,_n; picture _p; _p=nullpicture;
string _n[];
vardef _s(expr _t,_f,_m,_x,_y)(text _c)=
   addto _p also _t infont _f scaled _m shifted (_x,_y) _c; enddef;
_n0="ec-lmss12";
_s("co",_n0,1.00000,0.0000,-5.3134,);
_s("o",_n0,1.00000,11.3806,-5.3134,);
_s("rner",_n0,1.00000,16.9085,-5.3134,);
setbounds _p to (0,-5.3134)--(36.0726,-5.3134)--
(36.0726,0.0000)--(0,0.0000)--cycle;
_p endgroup
mpxbreak

File tmpgraph.bck has:

\long\def\mpxshipout#1{\shipout\hbox{%
   \setbox0=\hbox{#1}%
   \dimen0=\ht0 \advance\dimen0\dp0
   \dimen1=\ht0 \dimen2=\dp0  \ht0=0pt \dp0=0pt \box0
   \ifnum\dimen0>0 \vrule width1sp height\dimen1 depth\dimen2
   \else \vrule width1sp height1sp depth0sp\relax
   \fi}}
\mpxshipout{% line 3 test.mp
\ss coorner}
\end{document}
\end{document}


File tmpgraph.log shows context loading ec fonts (why not lm now)?
yours sincerely,
dr. H. van der Meer