A tough economy is weighing on growth.Trex (TREX -21.49%) used cost controls to deliver an earnings beat despite lower-than-expected revenue, but investors were more focused on the company's weak forecast for future demand.The stock is plunging post earnings, down 21% as of 1:30 p.m. ET.Lagging revenue in a tough marketTrex is a maker of composite decking and wood alternatives. The company is tied to the housing and remodeling industries, and its value proposition includes a higher up-front cost in exchange ...