How to add a vertical line to the chart

To add a vertical line to your line or scatter chart, do the following:
1. Add the date with the current date to your data, for example:

2. To add a new data series to your chart, do one of the following:
- On the Chart Design tab, in the Data group, choose Select Data:
- Right-click in the chart area and choose Select Data... in the popup menu:
In the Select Data Source dialog box, click the Add button and in the Edit Series dialog box, type:
- In the Series values box - the cell with the current date ($C$2):

3. If it is necessary, change data series:
- Right-click in the new data series and choose Change Series Chart Type... in the popup menu:
- In the Change Chart Type dialog box, choose the Scatter type:
In the Select Data Source dialog box, click the Edit button, and in the Edit Series dialog box, type:
- In the Series X values box - the cell with the current date ($C$2)
- In the Series Y values box - constant value (1):

4. Select the new data series, then do one of the following:
- Click on the Chart Elements button, select the Error Bars list, then choose
More Options...:
- On the Chart Design tab, in the Chart Layouts group, click the
Add Chart Element drop-down list:
In the Add Chart Element drop-down list, choose the Error Bars list, then click More Error Bars Options...:
Excel propose several error bars, also you can use More Error Bars Options.... If necessary, you can fine-tune the error bar settings from the Format Error Bars pane:

If you change the date, the chart will be changed too:

You can then make any other adjustments to get the expected look.
See also this tip in French: Comment ajouter une ligne verticale au graphique.