documentclass[tikz]{standalone}

usepackage{tikz,pgfplots}
pgfplotsset{compat=1.9}

begin{document}
begin{tikzpicture}

begin{axis}
coordinate (p1) at (1,1);
coordinate (p2) at (2,2);
addplot coordinates{(p1)(p2)};
end{axis}

end{tikzpicture}
end{document}

Runaway argument?
p1)(p2)pgfplots@EOI pgfplotsscanlinelengthcleanup
pgfplots@coord@stream@end
ETC.
Paragraph ended before pgfplots@loop@next was complete.
<to be read again>
               par
l.15