When zooming into canvas plots the resolution is sometimes limited.
See here: https://groups.google.com/forum/?fromgroups#!topic/comp.graphics.apps.gnuplot/M2ZOD445uWg
Dots appear on the same x position because the oversampling rate is to small for my application.
So I recompiled gnuplot (ubuntu precise) with an oversampling rate of 10000 to solve my problem.
From file canvas.trm:
Having a parameter to set this from command line would be great.