tests in log-scale plots

[John W. Eaton]

On  6-Sep-2011, Carlo de Falco wrote:

| the log-scale plotting functions (semilogx/semilogy/loglog) were removed from 
the test statistics with
| 
| ## Remove from test statistics.  No real tests possible.
| %!assert (1)
n| 
| actually a test is indeed possible, for example the following:
| 
| %!test
| %! a = logspace (-5, 1, 10);
| %! b =-logspace (-5, 1, 10);
| %! loglog (a, b)

I should have written a better comment.  Instead of "No real tests
possible", maybe I should have said something like:

  ## I can't think of any way to test this function that does not
  ## require a display and generates a plot.  We need to be able to 
  ## run the tests in environments where there is no display.  So
  ## instead of tests, we have to settle for demos.

Also, unless there is an unexpected error, your test doesn't catch
incorrect results without manual inspection, does it?  I think that if
a test can't catch incorrect results, then it should be a demo instead.

| catches a bug occurring in all the functions semilogx/semilogy/loglog in 
octave <http://savannah.gnu.org/bugs/?33249>, 
| while in matlab it is (almost) equivalent to
| 
| loglog (a, abs(b), 'ydir', 'reverse')
| set (gca, 'ydir', 'reverse')
| 
| woud it make sense to add the test above to semilogx/semilogy/loglog or maybe 
a demo like the following?

It's fine to add a demo.  But tests should only succeed or fail, not
produce any output or graphics.

jwe