Simple steps to reproduce:
1.) Create a Points-and-Lines Graph from one serie of data (i.e. create one line
graph)
2.) Set the graph to have smooth lines (Cubic spline, resolution 20)
3.) Finish the graph wizard and edit the graph
4.) Set the line width to 0,5cm and gice it a dark color
5.) Save the graph - the line is now ok, rendered well in the correct place
6.) Now edit the grap again, and set the line transparency to more than 0%
7.) Voila - the points are at the correct place, while the line gets shifted.
Leave the graph and the line disappears completely!