Pine Script - Mansfield Relative Performance indicator

study("Mansfield Relative Performance indicator")
// Mansfield Relative Performance indicator
//
// Source: http://stageanalysis.net/reference/how-to-create-the-mansfield-relative-strength-indicator/558
// Limits - This code is limited to the Daily value of the S&P 500 (SPX)
// Rewrite for Tradingview "Pine Script" by Kocurekc, April 2014
// https://getsatisfaction.com/tradingview/
// https://www.tradingview.com/u/kocurekc/
// Code is provided as public domain, no warranty

len = input(title="Lookback Period", type=integer, defval=50, minval=2)
SNP = security("SPX", "D", close)
RP = (close/SNP)*100
MRP = ((RP/sma(RP, len)) - 1 )*100
plot(MRP)