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 Series Property: Now expand series legend shape and select 'triangle' in the dropdown as shown below.

5. Now click on legend tab and Enter Following details in Legend Property:
Toggle Enable Legend
Disable Default Marker

6. Click on Preview and Save and Exit.

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