Follow below steps:
1. Create a line chart with required points.
2. Create a calculated field to find the avg for the points lying in the sheet.
AVG = WINDOW_AVG(SUM([Sales]))
3. Create another calculated field to find standard deviation of the points on the sheet.
SD = WINDOW_STDEV(SUM([Sales]))
4. Create another sheet to find the boundary line of the line chart to find outliers.
Upper = AVG + SD, Lower = AVG - SD .
5. Use created fields as reference lines and show the line chart to show outliers.
Hope this helps you.