WPP took another step forward in strengthening its digital media portfolio after announcing its subsidiary Kantar has invested in London-based market research firm ZappiStore.The PR giant now holds a stake in the analytics group, which was launched in 2013, as an automated data collection through a self-service platform and now plans to expand into 11 additional markets. The investment falls in place with WPP's plan to derive 40-45% of its revenue from digital in the next five years. The FTSE 100 constituent generated digital revenues of over $6bn in 2013.WPP did not mention any financial details of the investment, which it expects will strengthen its digital capabilities.As of 10:20 the share price had dropped by 0.3% to 1238p.WS