Elder Ray (Bull Power)

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//  Copyright by HPotter v1.0 23/07/2014
// Developed by Dr Alexander Elder, the Elder-ray indicator measures buying 
// and selling pressure in the market. The Elder-ray is often used as part 
// of the Triple Screen trading system but may also be used on its own.
// Dr Elder uses a 13-day exponential moving average (EMA) to indicate the 
// market consensus of value. Bull Power measures the ability of buyers to 
// drive prices above the consensus of value. Bear Power reflects the ability 
// of sellers to drive prices below the average consensus of value.
// Bull Power is calculated by subtracting the 13-day EMA from the day's High. 
// Bear power subtracts the 13-day EMA from the day's Low.
// You can use in the xPrice any series: Open, High, Low, Close, HL2, HLC3, OHLC4 and ect...
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study(title="Elder Ray (Bull Power)")
Length = input(13, minval=1)
hline(0, color=purple, linestyle=line)
xPrice = close
xMA = ema(xPrice,Length)
DayHigh = iff(dayofmonth != dayofmonth[1], high, max(high, nz(DayHigh[1])))
nRes = DayHigh - xMA
plot(nRes, color=blue, title="Bull Power", style = histogram)