documentclass[convert]{standalone}
usepackage{pgf}
usepgflibrary{plothandlers}
begin{document}
begin{pgfpicture}
  pgfplothandlerrecord{mystream}
  pgfplotstreamstart
  pgfplotstreampoint{pgfpoint{1cm}{0cm}}
  pgfplotstreampoint{pgfpoint{2cm}{1cm}}
  pgfplotstreampoint{pgfpoint{3cm}{1cm}}
  pgfplotstreampoint{pgfpoint{1cm}{2cm}}
  pgfplotstreamend

  pgfplothandlerlineto
  mystream

  pgfplothandlerclosedcurve
  mystream

  pgfusepath{stroke}
end{pgfpicture}
end{document}

pgfplothandlerrecord{<macro>}
pgfplotstreamstart
...
pgfplotstreamend

def<macro>{%
pgfplotstreamstart
...
pgfplotstreamend}