Create a Self-Expanding Chart

This happens very often with most of the Excel users. You have a set of data that you use to create a chart. If you latter want to add more data to the chart, you need to redefine your chart data series, unless if you didn’t add rows or columns in the middle of your original data where in this case, Excel will assume them on your chart.
To avoid this, just create your chart as usual. Then select any cell inside the range of data that you used to create the chart. Go to Insert-Tables to convert the range of data to a table. This is all you need to do for Excel to recognize the data as a table and the data added to the table can be reflected on the chart without the need to redefine your source.