Add optional scrolling to Chart like Telerik RadChart Scrolling:
http://www.telerik.com/help/silverlight/radchart-features-zooming-and-scrolling.html

And/Or add a paging feature to the Line Chart that allows the user to page from right to left through data. Use the paging feature of the datasource to allow huge datasets to be viewed in increments (querying pages of data for the chart as the user pages from right to left) for performance.  For example, if a years worth of hourly data is requested, page by month through the chart and query the data from the datasource one month at a time.  This could work nicely with a grid configured for paging using the same datasource to display the data and chart in sync on the page.