Shinystat uses a mobile average over 7 days. On the chart, the mobile average is displayed as a trend line which smoothes the recurrences of the days and provides you with a quick overview of the period trend.
Formula:
Yn = (Xn-3 + Xn-2 + Xn-1 + Xn + Xn+1 + Xn+2 + Xn+3) / 7