This is an interesting illustration of the company's improving returns even in spite of its cost leadership approach to business. However, not even this paints the entire story. Return on equity is driven by net income and book value of equity. For this reason, it is possible that debt can inflate a company's return on equity by decreasing a company's total common shareholder's equity, or that it can deflate it by decreasing said company's net income.
To account for these differences, I factored out both the long-term debt and tax-adjusted interest expenses of Wal-Mart, Costco and Target in the graph below: