By Dan Weil Morningstar took the stocks its analysts view as undervalued and screened for those with the largest fair-value increases. As you well know, stocks have tumbled, with the S&P 500 dropping 20% year to date. But some companies still are going strong. Enough, in fact, that in the second quarter, Morningstar analysts increased their fair-value estimates by 10% or more for 52 of the 859 U.S.-listed companies that they cover. To be sure, that’s barely more than half the quarterly average of 98 in the past five years. But it still gives investors quite a few stocks to consider. Looking at…