Due to the recent changes in the Yahoo historical data stream (see http://sourceforge.net/forum/message.php?msg_id=4128620\) I have published an update for the Yahoo plugin that fixes these problems.
The update is available from the Update site:
In EclipseTrader open Help -> Software Updates -> Find and Install. Select 'Search for updates of the currently installed features' and click Finish. You should see the mirror selection dialog for the Eclipse RCP feature and, after selecting on of the mirrors, the list of updated features. The EclipseTrader feature should be already selected, if not, select it. Ignore the Eclipse 3.2.1 patch update it isn't necessary and may cause problems. Click on Finish and install the updated plugin. Restart EclipseTrader and all should work well.
To correctly see the chart data, you need to delete the old, incorrect, data. To do so you can either delete the contents of the workspace/history directory (close EclipseTrader first) and update all charts, or you can open the chart settings (Edit -> Settings...), click on 'Clear chart data', save, and update the chart.