Microsoft has announced the purchase of Visio Corporation, a specialist in technical diagramming software, in a $1.3 billion stock deal. The purchase will help Microsoft strengthen its grip on the market for business productivity software, already dominated by its Office suite of word processing, spreadsheet and presentation products. Visio software lets professionals such as corporate administrators and designers make flow and organisational charts and technical drawings such as engineering schematics. Visio was founded in 1990, and recorded $166 million in sales last year.