It's plot the with a upper and a lower band.
The bands are calculated like this:
UpperBand = 20 + 20% *
LowerBand = 20 - 20% *
But, you can change the paramters. I use the 200SMA with 100% of either.
How to use:
If the price are negociating out of the bands, it's dangerous to trade this stock.
Hope you like it.