Positive Volume Index Strategy

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.

WARNING:
- This script to change bars colors.