Yeah, Glenn originally reported it as 5.1.0, I moved it to dev, but feel free to keep it 5.1.0 and fix on stable if you think it's important.

This is probably my doing.  I re-worked line and _line_ quite a bit.  Assigning the bug to myself.

This is marked against the development version of Octave.  Is it also present on the new 5.1.0 stable release?

First, thanks to all the Octave contributors that make Octave so great.

When using line(...) to plot into an existing axes handle, an undesirable axes is created in the current figure when no axis exists.  Run attached unwanted_axis.m for demonstration.

This occurs for Open_GL graphics toolkit as well as ftlk toolkit.

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GNU Octave Version: 5.1.0 (hg id: d05d6eebde10)
GNU Octave License: GNU General Public License
Operating System: MINGW32_NT-6.1 Windows 7 Service Pack 1 x86_64
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This error occurs when _next_line_color_.m and _next_line_style_.m eventually get called.  In these m-files gca() is called, which creates an axis when one doesn't exist in the current figure (even though the explicit handle to the desired axis is passed to line). To remedy, _line_.m can be modified to pass the desired axes handle to _next_line_style_.m and _next_line_color_.m.  Both of these functions need to be modified to allow passing in the desired axis handle as well.

Diffs for a very crude hack to address this are attached.