This indicator is a ported MQL5 indicator from http://www.mql5.com/ru/code/518 (in Russian).
Formulas:
PVI[i]=PVI[i-1], if Volume[i]<=Volume[i-1] and
PVI[i]=PVI[i-1]*(1+(Close[i]-Close[i-1])/Close[i-1]), if Volume[i]>Volume[i-1].
Download:
Negative Volume Index is available here.
viewtopic.php?f=17&t=9707&p=113
Volume Index is combination of two.
viewtopic.php?f=17&t=65176
The indicator was revised and updated