Go ahead nd use combo chart you can achived this by writing 2 expressions first expression will be showing your data in bar chart and use second expression for refrence line.
for first expression display option in the expression tab should be bar and for the scond expression it should be line.
1st exp sum(Sales)
2nd exp sum(if([Month Num]<=6,25,35))
Hope this will help you........!!
1.PNG 17.0 K