Volume At Price histogram is also known as "Volume Profile" chart.
To turn it on simply go to Tools->Preferences and change Type of the VAP from "NONE" to "Left-side solid area chart, behind" for example
VAP shows total volume of trading that occurred at given price level. VAP is calculated from data bars that are currently visible.
Actual algorithm involves not ONE price but High-Low price RANGE.
AmiBroker DISTRIBUTES equally bar's volume over High-Low range to produce
VAP histogram. For example if bar's volume is 10000 and H-L range spans 3 'lines" of
VAP histogram than each of 3 lines
involved gets added 10000/3 to produce statistics. This gives much more accurate results than using single price
as some other implementations do.
To turn VAP on/off use: Tools->Preferences->Main chart
You can also add VAP to your own custom charts using PlotVAPOverlay AFL function.