Seems as if this is (was) a bug in pgf/tikz: on my machine, I get a straight line.

I can confirm that this happens with pgf 2.10 and the stable version(s) of pgfplots.

It vanishes for pgf 2.00 and/or for pgf CVS.

I have just uploaded an unstable version of pgfplots which incorporates the pgf 2.10 bugfix: your example will work with the unstable pgfplots version from http://pgfplots.sourceforge.net/ and pgf 2.10 as well.

So, to solve the problem in your application, I see the following approaches:


use the unstable of pgfplots (which is pretty stable now)
use pgf CVS
downgrade to pgf 2.00
use \addplot gnuplot instead and enable the shell-escape feature.


Note:
Installing pgf CVS or pgf 2.00 manually is more complicated than installing pgfplots since pgfplots comes in a TDS structure (which is compatible with local texmf installation directories). The pgf versions need to be adjusted manually before TeX distributions accept them as TDS.

PGFplot producing "noise" (incorrect graph)
Is it possible to update the current relative coordinate in TikZ without actually adding anything to the picture? For instance,

\node at ++(1, 0) (node1) {...};


will update the current x coordinate by incrementing it by 1, but how can I do this without actually adding a node? Also, it is possible to increment the position within a loop (i.e. to keep the new value between the iterations)?
