Volume At Price histogram is also known as "Volume Profile" chart.
To turn it on, simply go to Tools->Preferences and change the VAP Type from "NONE" to "Left-side solid area chart, behind" for example.
VAP shows total volume of trading that occurred at a given price level. VAP is calculated from data bars that are currently visible.
The actual algorithm involves not one price, but a High-Low price range.
AmiBroker equally distributes the bar's volume over the High-Low range to produce a
VAP histogram. For example, if a bar's volume is 10000 and the H-L range spans 3 'lines' of the
VAP histogram, then 10000/3 is added to each of the 3 lines
involved to produce statistics. This gives much more accurate
results than using a 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.