A Volume indicator is the trading volume for current bar. Unit of volume for stocks is in million shares. Volume for index, futures is counted by ticks (trades).
Weight of the histogram bars cannot be formatted. The color of the histogram bars can be formatted. Volume indicator is defaulted in subgraph 2. However, you may change it to subgraph 2 or 3 if subgraph 2 is used by other indicators.
Average volume is calculated, blue line default length 20. It will show trend of the trading volume, when price is up, the volume is up or down.