To add a dotted forecast line in an existing line chart in Excel, please do as follows:

1. Beside the source data, add a Forecast column, and list the forecast sales amount as below screenshot shown.
Note: Please remember to add the sales amount of Jun in the Forecast column too.

2. Right-click the line chart, and click Select Data in the context menu. See screenshot:

3. In the Select Data Source dialog box, please click the Add button in the Legend Entries (Series) section. See screenshot:

4. Now the Edit Series dialog box comes out. Please (1) type Forecast in the Series name box, (2) specify the Forecast column excluding the column header as Series values, and then (3) click the OK buttons successively to close the two dialog boxes. See screenshot:

5. Now the forecast line is added to the line chart. Right-click the forecast line, and click Format Data Series from the context menu. See screenshot:

6. In the Format Data Series pane, please (1) click the Fill & Line icon, (2) expand the Line section; and (3) Select Round Dot from the Dash type drop-down list. See screenshot:

Note: If you are using Excel 2010 or earlier versions, it will open the Format Data Series dialog box. In the dialog box, please click Line Style in the left bar, select Round Dot from the Dash type drop-down list, and then close the dialog box.

So far, we have added the dotted forecast line in the existing line chart. See screenshot:

If you have Kutools for Excel installed, you can apply its Forecast Chart feature to quickly create a line chart which shows the actual values with solid line and display the forecast values with dotted line in Excel.

1. Prepare the source data containing both actual values and forecast values in two columns, and select the source data.

2. Click Kutools > Charts > Category Comparison > Forecast Chart to enable this feature.

3. Now the Forecast Chart dialog comes out. By default, the ranges are filled automatically into the Axis label range, Actual value range, and Forecast value range boxes based on the selected source data. If they are incorrect, correct them manually. Then click the Ok button.

4. A dialog pops out and tells you a hidden sheet is created to store the intermediate data. Click Yes to go ahead.

Now a line chart is created. In the chart, the solid line part presents the actual values, while the dotted line part shows the forecast values.

Notes:
In the Forecast Chart dialog, if you have ticked the Auxiliary guides option, typed in or referred a cell in the below box, it will create a forecast chart with a horizontal line as below screenshot shown.

How to add a single data point in an Excel line chart?