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)?


Question: PGFplot producing "noise" (incorrect graph)
Answer: 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.