add a description for the blue liner in the cost anomaly graph

https://doitintl.slack.com/archives/C2TG2E30C/p1609263459262000 High-level: The blue line shows the predicted cost plus allowing some deviation based on the previous week. Thus the expected cost. Detailed: The blue line shows the predicted cost (average cost) at a particular hour for a particular service plus 5 times the standard deviation of the previous week.