Here's how I did it.
Steps:
1. addd a tablix to your report.
2. Add Company, PurchaseOrder, PurchaseDate fields to the tablix.
3. Right-click the body line (with the lines).
4. Add Group: Parent Group
Group by: Company
Check Add group footer
Click Okay.
5. Now we have company1, Company, Purchase Order, Purchase Date.
Delete the leftmost column (Company1).