How to Mark Time Intervals on X-axis of Graph View

While in the Graph View, you can view data for a specific time for closer inspection:

To view data for a specific time, select a specific point in the graph area with the left mouse button. This will display an x-axis (vertical line).
To view specific data at the selected point, click and hold the left mouse button and drag the cursor to mark the time interval; then release the mouse button.
Click the Zoom icon to view the data in the marked time interval.