Optionally it can also be used to display visibility phases and statistical errors instead or in addition to the plot of amplitudes. See below for details. Where multiple spectral-line channels are selected from a given IF, the visibility is the average of those channels and is displayed at the corresponding mean UV radius. It is often possible to get a vague idea of how the map will appear from looking at this plot. Unresolved point sources appear as flat amplitude distributions.
Resolved sources display one or more gaussian fall-off distributions.
In some cases you may be able to get a better impression by running the alternate 'projplot' command. See help projplot for details. If the distribution is reasonably smooth, it is also possible to see the effects of poor calibration. The highlighting function is invaluable here. Highlighting each station in turn will eventually reveal the identity of a bad station or baseline.