Display 2nd measure data in a same line in chart

[qlik_venu]

Hi,

We have a line chart which shows a percentage measure. ( attached the chart).

There are 2 years for comparision.

We also need to show the difference in these 2 year values by a third imaginary line - present in the top ( 2 , 2, 2, -4).

Can you please suggest how to get the top line which indicates the data difference in a straight lien though the actual measure value changes.

thanks

Venu