[metapost] bullet placement: document mpgraph.pdf

[Yogeshwarsing Calleecharan]

Hi,
     I am looking at the document mpgraph.pdf:

http://www.tug.org/docs/metapost/mpgraph.pdf

and on page 3 it is shown how to just have bullets instead of lines in a 
graph. In particular, for proper placement of the bullets at the 
coordinates centre the following command is given:

def MPbullet =
btex \lower\fontdimen22\cmsy \hbox to 0pt{\hss\cmsy\char15\hss} etex
enddef;

and then followed on the top of page 4 by:

gdraw "agepop91.d" plot MPbullet

However this is not working. I have the lates TexLive and Metapost 
1.504. Here is the error message:

This is MetaPost, version 1.504 (kpathsea version 6.0.1)  9 JUN 2012 11:43
**plot.mp
(mpost.mp (c:/texlive/2011/texmf-dist/metapost/base/plain.mp
Preloading the plain mem file, version 1.004)) (./plot.mp
(c:/texlive/2011/texmf-dist/metapost/base/graph.mp
(c:/texlive/2011/texmf-dist/metapost/base/marith.mp
(c:/texlive/2011/texmf-dist/metapost/base/string.mp))
(c:/texlive/2011/texmf-dist/metapost/base/format.mp
(c:/texlive/2011/texmf-dist/metapost/base/string.mp)
(c:/texlive/2011/texmf-dist/metapost/base/texnum.mp)))
(c:/texlive/2011/texmf-dist/metapost/base/sarith.mp)
 >> plot.mp
 >> plot.mpx
! Unable to make mpx file.
l.7 btex
          \lower\fontdimen22\cmsy \hbox to 0pt{\hss\cmsy\char15\hss} etex
The two files given above are one of your source files
and an auxiliary file I need to read to find out what your
btex..etex blocks mean. If you don't know why I had trouble,
try running it manually through MPtoTeX, TeX, and DVItoMP


Can anyone please suggest how to fix this?

Thank you.

Best Regards,
Y.Calleecharan