declare lower;
input Period = 21;

plot TickIndex = close("$TICK");
#You can change the index by changing the symbol between the " ".  
#$TICK = NYSE Index
#$TIKQ = NASDAQ Cumulative Index
#$TIKRL = Russell 2000 Index
#$TIKSP = S&P 500 Index
#$TIKI = DJIA Index
#$TIKUS = All US Stock Index
 
TickIndex.SetPaintingStrategy(PaintingStrategy.HISTOGRAM);
TickIndex.DefineColor("Up", Color.UPTICK);
TickIndex.DefineColor("Down", Color.DOWNTICK);
TickIndex.AssignValueColor(if TickIndex > 0 then TickIndex.color("Up") else if TickIndex < 0 then TickIndex.color("Down") else GetColor(4));
TickIndex.SetLineWeight(3);

def StDev = stdev(data = TickIndex, length = period);
def AvgLine = Average(data = TickIndex, length = period);

def UpperDev = 2.0;
def LowerDev = -2.0;

plot UpperBand = AvgLine + UpperDev * StDev;
Upperband.SetDefaultColor(color.RED);
plot LowerBand = AvgLine + LowerDev * StDev;
LowerBand.SetDefaultColor(color.GREEN);

plot BaseLine = 0;
BaseLine.SetDefaultColor(color.GRAY);
plot Overbought = 800;
Overbought.SetDefaultColor(color.GRAY);
plot Oversold = -800;
Oversold.SetDefaultColor(color.GRAY);