Hi,
Let's suppose that the x-values(Category axis) and the y-values are in say
A2:A101, and B2:B101 respectively, and also that you have already created a
Line Plot with B2:B101 vs A2:A101.
In C2 enter, =B2
Now select C2:C4 (note that C3 and C4 are blank), move the cursor to the
bottom-right corner of the selected area (the cursor-icon will turn into a
'+' symbol), hold the left mouse-button, and drag down to C101. This action
will copy every third y-value into Column C.
Now add a series to your Line Plot, with C2:C101 vs A2:A101. Format the
second series as you wish to distinguish it from Series 1.
Regards,
B. R. Ramachandran