lines in the graph

The picture of the graph is useful to show the very subtle problem you are concerned about. But a copy of your Igor experiment file will let us examine the details of how you constructed the graph.

Thank you for your answer,

enclosed you will find the igor file

Thank you for the example file- it confirmed what I suspected. The faint lines are the edges of fill-to-zero areas. I don't know why they would appear (but I have a suspicion) and I consider it a bug. But in the meantime I think I have a better way to create that graph, using fill to next instead of fill to zero.

I used Graph->Reorder Traces to move the Re_4f_1_Re_4f_4 trace to the back (top of the list) just before Background_Re_4f_4. Then I changed the Grouping mode of Re_4f_1_Re_4f_4 to "Draw to Next", and changed the mode of Background_Re_4f_4 to Lines Between Points mode instead of Fill to Zero.

I have attached my version of your experiment file.