
Explanation:
Step 1: Create a 12-month...
You can use calculated measure to get the expected result.
1. Create a calculated column for the date.
2. Create a measure for 12 months moving average.
3. Drag the Line Chart into your canvas as below. (step 2 below)

Step 2: Select the line chart
Step 3: From the Analytics pane, add a Median line
Reference:
https://community.powerbi.com/t5/Desktop/Moving-Average/td-p/43041
Recent Comments (The most recent comments are at the top.)
1.Select the line chart.
2.From the Analytics pane, add an Average line.
3.Turn on data labels for the new line.