Sets or returns whether the label will contain the R-squared value for the Trendline's equation.
The RSquared value is dynamically calculated based on the current values in the trendline's Series
. Since it is dynamically calculated by Excel, the actual RSquared value is not accessible through the ExcelWriter API. This property only adjusts the visibility of the value within the trendline's label. If you wish to adjust the formatting of the RSquared value, use the Trendline.TrendlineLabel