A solution to this problem may be found in plotting via interval arithmetic.
For example see my paper on Honest Plotting...
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1.8459
Since Maxima does not have an officially installed interval arithmetic
package, this could be done by loading in one of several
lisp-language interval packages (I have at least two), extending it
to cover all functions with numerical methods, and integrating it into
the Maxima user-level context.
This solves the
problem of the discontinuity-too-narrow-to-notice, too.
There are some follow-up papers on this topic from other people.
RJF