(Reuters) - Advertising giant WPP has agreed to sell a 60% stake in data analytics arm Kantar to private equity firm Bain Capital, in a deal that values Kantar at about $4 billion (£3.2 billion) and gives WPP funds for its turnaround plan.WPP, the owner of advertising agencies including Ogilvy and Wunderman Thompson, is in the midst of an overhaul following several profit warnings and the abrupt departure of founder Martin Sorrell over a complaint of misconduct, which he denies.WPP said..