From MSDN:
The FastLine chart type is a variation of the Line chart that significantly reduces the drawing time of a series that contains a very large number of data points. Use this chart in situations where very large data sets are used and rendering speed is critical. Some charting features are omitted from the FastLine chart to improve performance. The features omitted include control of point level visual attributes, markers, data point labels, and shadows.
eg. If you are going to display tens or hundreds of thousands of data points.
Link: http://msdn.microsoft.com/en-us/library/dd489249.aspx
Paul sasik
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