Because of my fiddling with implicit functions in order to draw algebraic curves (see Plotting an implicit function using TikZ and How to use the results of raw output to create a smooth plot), I'm wandering though the source code of pgfplots.

My main concern is the file /texmf/tex/generic/pgf/modules/pgfmoduleplot.code.tex which contains a definition of \pgf@parsexyline. Trust me, after a few minutes you start believing Till Tantau built PGF/TikZ on sexy lines.

For people looking at how to draw implicit functions using pgfplots, I refer you to Howto: Draw Algebraic curves using TikZ.