Rsquared
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The rsquared indicator calculates how well the price approximates a linear regression line. The indicator gets its name from the calculation, which is, the square of the correlation coefficient (referred to in mathematics by the Greek letter rho, or r). The range of the rsquared is from zero to one.
... Courtesy of FMLabs
RSquared(int period)
RSquared(ISeries<double> input, int period)
Returns default value
RSquared(int period)[int barsAgo]
RSquared(ISeries<double> input, int period)[int barsAgo]
double; Accessing this method via an index value [int barsAgo] returns the indicator value of the referenced bar.
input 
Indicator source data (?) 
period 
Number of bars used in the calculation 

You can view this indicator method source code by selecting the menu New > NinjaScript Editor > Indicators within the NinjaTrader Control Center window.