There are few options how you can do that.
If I understood you correctly, you will need to have a report based on Weekly dimension.
One of the options: Create a new field from your "Deal Date" field as Week() function, for example
Week([Deal created]) as Weekly_Deal_Created_Dim
Use the same Expression (sum(Deal_Count), or else... ) and use the Weekly_Deal_Created_Dim as your chart's dimension.
So this approach requires extra steps, but it's reliable and very good performance-wise.
If script modification is not applicable in your case, use a Week([Deal created]) as Calculated dimension in your chart.
Should work fine too, but chart's performance will not be as good as with the approach above.
P.S. When applicable, please mark this answer Correct or Helpful.