You mentioned that you have defined a variable!
Then you want to link it to your chart
What I will do in this case is to right click on the chart > Properies > Expression > Add new expression (click ADD) > then enter Relative * (1+ VARIABLE NAME)
Then give the new expression a name LABEL
You will see new column added to the straight table with the label you've given
then right click on the blank space on the page, select Slider > in the General tab > check the radio button for variable and select your variable name (if not showing by default) > down the page set minimum value & maximum value > select the Number tab and specify % > click OK and Apply
Let me know if that helps.