Dear WE,
many thanks for your help. I shall confront your attached file with mine a little more, to understand how it works.
But let me say that you are absolutely right in stating - contrary to what I wrote in my first post - that it is not enough in MC 15 to insert the two vectors. One needs, as you say, to turn at some point the plot to "scatter plot". The example that inspired me was from: http://communities.ptc.com/docs/DOC-2488#comment-6098
and probably the plot in that example was already "cured" against the bug as you suggested.
Tweaking the plot in that way makes also v.11.2 working (in v.11.2 Scatter plot is called Data plot)!