Calculates the Exponential Weighted Moving Average (EWMA) control chart. In this case UCL and LCL are constant (asymptote) limits. This chart is also known as exponentially smoothed or geometric moving average chart. It evaluates the process level using an exponentially smoothed moving average. Here, by the term exponentially, we mean the procedure by which individual observations or subgroups are given progressively less importance or weight. When compared to the XChart, the EWMA chart is more sensitive to smaller shifts in the process level. 

The exponentially weighted moving average is defined as

XHat[t] = r*XHat[t] + (1-r)XHat[t-1] ,

where r is a constant and XHat[t] are EWMA chart points. The starting value for the first sample at time t = 1 = XHat[0] is grand mean value.