Online grocery firm Ocado was the standout performer on the second-tier FTSE 250 index on Monday afternoon, rising to his highest ever level since it debuted on the stock market in July 2010. While there's no newsflow, one trader told Sharecast that today's moves are likely a result of 'short-sellers' continuing to cover their positions.The stock has now more than tripled in price since the start of the year. It is up over 100% since announcing its new tie-up with Morrison at the end of April to help the supermarket develop an internet shopping channel.Infrastructure group Balfour Beatty rose after proposing to close a number of regional offices as part of an internal review of its UK construction business. The company also announced that it had been awarded a contract worth over £110m to construct a 43-storey tower in London, the tallest building constructed by the firm in the UK.Betfair was also higher after Citigroup upgraded the stock from 'neutral' to 'buy', with the broker saying that the online gaming group's valuation looks attractive compared with the wider sector. Mining stocks were sharply lower today as a host of economic data from China painted a gloomy picture for the world's second-largest economy. In particular, export growth fell to a 10-month low in May which was weighing on the outlook for commodities demand. African Barrick Gold, Ferrexpo and Lonmin were among the worst performers.FTSE 250 - RisersOcado Group (OCDO) 308.90p +12.00%Balfour Beatty (BBY) 226.80p +5.00%Intermediate Capital Group (ICP) 452.70p +3.62%Betfair Group (BET) 836.50p +3.02%Sports Direct International (SPD) 531.50p +2.90%BBA Aviation (BBA) 278.60p +2.46%Savills (SVS) 598.50p +2.31%Alent (ALNT) 355.90p +2.24%888 Holdings (888) 154.90p +2.24%Dunelm Group (DNLM) 916.50p +2.17%FTSE 250 - FallersGrainger (GRI) 145.60p -4.21%African Barrick Gold (ABG) 134.00p -4.15%Ferrexpo (FXPO) 154.50p -3.38%Premier Farnell (PFL) 202.70p -3.25%Daejan Holdings (DJAN) 3,810.00p -2.98%Lonmin (LMI) 288.60p -2.86%Electrocomponents (ECM) 255.30p -2.85%Hays (HAS) 88.30p -2.70%TalkTalk Telecom Group (TALK) 221.60p -2.68%Domino Printing Sciences (DNO) 634.00p -2.46%