[Octave-bug-tracker] [bug #60588] Vector/scalar combination in plotting

[Michael Leitner]

URL:
  <https://savannah.gnu.org/bugs/?60588>

                 Summary: Vector/scalar combination in plotting
                 Project: GNU Octave
            Submitted by: mleitner
            Submitted on: Wed 12 May 2021 06:25:01 AM UTC
                Category: Plotting
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Documentation
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 6.1.0
         Discussion Lock: Any
        Operating System: Any

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Details:

Plotting


x=(1:10)';
y=1;
plot(x,y)


gives ten points of different colour. I would have expected the y to be
singleton-expanded and thus a single line to be plotted. The point is that if
x is a very large vector, the execution takes unexpectedly long, while a
single line would have been drawn much quicker. I see that the Matlab
documentation says that in the present case discrete points are plotted, but
they say nothing about whether they should have the same style or not (which
probably would also complete faster than the present case). As I see it, the
octave documentation does not treat the present case at all. 

So this is at least a documentation bug. Perhaps it is also a Matlab
incompatibility, if Matlab should have the same style on all points. What I do
not get is their point that to see the points a marker symbol has to be
specified -- if no symbol is specified, does the plot stay empty? Because in
fact that is what happens if in octave I execute plot(x,y,'-'), presumably
because it plots ten lines consisting only of a single point. Thus, for the
simplest conceivable plot you can request, just a single horizontal line, you
have to do something like


plot([0 1],[1 1])


while the more obvious and simple


plot([0 1],1)


does not work, not even with an '-'. That's why I would argue here even for a
voluntary Matlab incompatibility.




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