You need to do some changes in data model.
You need to link both the tables and then this will be easier for you.
The script will look something like this.
Load Month_Year,Qty as Item_Inquiry_Qty From ITEM_INQUIRY;
Load Month_Year,Qty as BACKDATE_Qty From BACKDATE;
Here both the tables will link by Month_Year.
Now all you need to do is create a chart with expression as
sum(Item_Inquiry_Qty) + Sum( BACKDATE_Qty)
Hope this is clear.