Negative Volume Index (NVI)

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//  Copyright by HPotter v1.0 11/06/2014
// The theory behind the indexes is as follows: On days of increasing 
// volume, you can expect prices to increase, and on days of decreasing 
// volume, you can expect prices to decrease. This goes with the idea of 
// the market being in-gear and out-of-gear. Both PVI and NVI work in similar 
// fashions: Both are a running cumulative of values, which means you either 
// keep adding or subtracting price rate of change each day to the previous day`s 
// sum. In the case of PVI, if today`s volume is less than yesterday`s, don`t add 
// anything; if today`s volume is greater, then add today`s price rate of change. 
// For NVI, add today`s price rate of change only if today`s volume is less than 
// yesterday`s. 
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study(title="Negative Volume Index", shorttitle="Negative Volume Index")
EMA_Len = input(255, minval=1)
xROC = roc(close, 1)
nRes = iff(volume < volume[1], nz(nRes[1], 0) + xROC, nz(nRes[1], 0))
nResEMA = ema(nRes, EMA_Len)
plot(nRes, color=red, title="NVI")
plot(nResEMA, color=blue, title="EMA")