IMPLOT2 may be used to plot implicit functions of exactly two variables.
It uses a similar approach to IMPLOT (by Arthur Jutan), but extends the capabilities of IMPLOT to allow the specification of one or more lineseries properties: e.g. line width, line color, line style etc. It can also return a matrix containing all plotted datapoints, with contiguous line segments clearly demarcated.
Note that IMPLOT2 obtains isocontours for f(x,y)=0 via a fairly crude approach, viz. using the low-level CONTOURC function. Thus it is not ideal for applications where high levels of accuracy are required.