1. Click on hamburger icon.

2. Click on visualization tab and open Line Chart in edit mode.

3. The Edit menu box appears as you click on Line chart icon. And a widget gets added in content place automatically in dashboard.

4. In Formatting Property click on Value Axis Property.

5. Enter Following details in Value Line:
Border Width: 2
Border Color: #000000
Border Opacity: 0.6

6. Fill above details in edit box, As Shown in figure:

7. Click on Preview and Save and Exit.

8. Now, match your Line chart with the below example, it should look like the figure below: