I have a number of points out of a countour plot of a circle. Unfortunately the points are ordered by coordinate position. If I plot the data with "lines" as style data, gnuplot draws lines through my circle. However, I only like to have the circle. How I can change the configuration of gnuplot or do I have the change the order of my points?

As Bernhard commented, this is generally not something you can do easily in Gnuplot. However for your particular issue there's a particular solution. If the points are ordered say by xcoordinate, you can plot first those with a ycoordinate greater than the ycoordinate of the center of the circle, and then in a separate command those with a lower ycoordinate. E.g, for a center on xaxis:
Connecting the two arcs can be done manually using this answer. It could also be done automatically, you would need to save the first and last occurences (gnuplot 4.4 and greater) 

