A new property for the Spark Chart

I think a StartTime would be helpful for the Spark Chart control so it can display historical data without also having to display real-time data.

I have a Value Box that displays a historical maximum over the period of a month. I'd like to include a chart of the data that corresponds to it (to allow the user to judge if the value is an outlier or actually representative of the data). The Spark Chart allows me to select a TimeDuration, but not a StartTime that can match the AggregateStartTime of the Value Box. I have to use the Trend Graph instead. However, I change the SourceTag on the Value Box with another control. It would get confusing if a user changed the SourceTag of the Value Box, but did not change the trend that is displayed on the Trend Graph as well.