Click the View Mode button in the Watch Window toolbar 
to put the Watch Window cursor into View Mode.
In this mode, the cursor will show values of data points while mousing over them. By clicking on data plots you can mark live data with Timestamps.
The other cursor mode available in the Watch Window is Select Mode, which is used to select sections of data to be exported.
See also
- Adding notes to timestamps
- Creating and deleting timestamps in the Watch Window
- Marking Times with Timestamps in the Watch Window
- Navigating between timestamps in the Watch Window
- Toggling timestamp value labels