June 23 (Reuters) - Canada’s main stock index edged higher at open on Wednesday as heavyweight energy stocks advanced, while data showing a plunge in domestic retail sales in April and May capped the gains. * At 9:30 a.m. ET (13:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was up 16.77 points, or 0.08%, at 20,217.42. (Reporting by Amal S in Bengaluru; Editing by Sriraj Kalluvila) Our Standards: The Thomson Reuters Trust Principles.