Copy and paste it! Say you have a chart with the months on the x-axis (horizontal). As you add a new month to the data table, you can select the applicable column (or columns to add several months) and copy. Be sure to include the column title. Then select your chart and paste. Your chart should include the new data.
You can do this across workbooks and worksheets. Be sure to structure the data the same as the original chart data to avoid unpleasant surprises! Undo (Ctrl+Z) works fine here if you make a mistake.