This indicator takes the standard ATR and expresses it as a percentage of the OHLC4 price. This has the advantage of normalising the ATR value across the history of an asset. For example, an ATR of value 20 when the price is 2000 actually has a very different meaning when the price rises to 4000. The ATR may be the same value but actually the volatility it represents has halved.
I also add an SMA to the value and a histogram which shows the difference between the two. Positive values mean that volatility is expanding while negative values mean volatility is contracting.