A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
One way is to build the chart to show 50 values and then hide the rows (or columns) containing the data for the points that you don't want to appear.
If that is not convenient, you can create dynamic range names that you can use in the series definition. For an example see http://peltiertech.com/Excel/Charts/DynamicColumnChart1.html