Hi there!

To accomplish this, you can use a combination of Excel's chart formatting options and a dynamic named range. Here are the steps:

1. First, create a dynamic named range for your data. This will allow your chart to automatically update as new data is added. To do this, select your data range (including headers) and go to Formulas Define Name. In the "New Name" dialog box, give your range a name (e.g. "DataRange") and in the "Refers to" field, enter the formula
   Formula:

   "=OFFSET(Sheet1!$A$1,0,0,COUNTA(Sheet1!$A:$A),2)" 

   . This formula will automatically adjust the range based on the number of rows of data you have.
   
2. Create a line chart using your named range as the data source. To do this, select your data range and go to Insert Charts Line Chart. Excel will automatically create a line chart with your data.
   
3. Right-click on the chart and select "Select Data". In the "Select Data Source" dialog box, click on the "Edit" button next to the "Legend Entries (Series)" box.
   
4. In the "Edit Series" dialog box, select the series you want to format (in this case, the entire line) and click on the "Format" button.
   
5. In the "Format Data Series" dialog box, go to the "Line" tab and select a solid line style for the first portion of the line (actual data). Then, go to the "Marker" tab and select "None" for the marker options.
   
6. Click "OK" to close the "Format Data Series" dialog box, then click "OK" again to close the "Edit Series" dialog box.
   
7. Right-click on the chart again and select "Select Data". In the "Select Data Source" dialog box, click on the "Add" button next to the "Legend Entries (Series)" box.
   
8. In the "Edit Series" dialog box, enter a name for the new series (e.g. "Forecast"), then select the range of forecasted data (excluding the actual data) using the "Series values" box. Click "OK" to close the dialog box.
   
9. In the "Select Data Source" dialog box, select the new "Forecast" series and click on the "Format" button.
   
10. In the "Format Data Series" dialog box, go to the "Line" tab and select a dotted line style for the forecasted data. Then, go to the "Marker" tab and select "None" for the marker options.
    
11. Click "OK" to close the "Format Data Series" dialog box, then click "OK" again to close the "Select Data Source" dialog box.

Your chart should now display the actual data with a solid line and the forecasted data with a dotted line. As you add new data, the chart will automatically update to reflect the new actual data and adjust the formatting accordingly.