1. Click on hamburger icon.

2. Click on dashboard 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 Category Axis Property.

5. Enter Following details in Category Labels:
Toggle Enable Labels
Toggle Label Position Top
Toggle Label Inside
Alignment: Rotate 90
Label Placement: On Ticks

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: