The plot package provides simple interface for laying out a plot and provides primitives for drawing to it.
The plotter package provides a standard set of 'Plotters' which use the primitives provided by the plot package for drawing lines, scatter plots, box plots, error bars, etc. to a plot. You do not need to use the plotter package to make use of gonum/plot, however: see the wiki for a tutorial on making your own custom plotters.
The plotutil package contains a few routines that allow some common plot types to be made very easily. This package is quite new so it is not as well tested as the others and it is bound to change.
The vg package provides a generic vector graphics API that sits on top of other vector graphics back-ends such as a custom EPS back-end, draw2d, SVGo, and gopdf.
You can get gonum/plot using go get:
go get github.com/gonum/plot/...
If you write a cool plotter that you think others may be interested in using, please post to the list so that we can link to it in the gonum/plot wiki or possibly integrate it into the plotter package.