rlocus/gnuplot problem

[A. Scottedward Hodel]

I'm using the recently updated rlocus function in octave 2.9.10 and have found a problem that I don't know how to fix.

octave:2> rlocus(tf(1,[1,2,10]))

ans = [](0x0)

octave:3>

gnuplot> plot "-" using ($1):($2) title "" with lines linestyle 1, "-" using ($1):($2) title "asymptotes" with lines linestyle 2, "-" using ($1):($2) title "locus" with lines linestyle 3, "-" using ($1):($2) title "" with lines linestyle 4, "-" using ($1):($2) title "open loop poles" with points linestyle 5;

                                                                                                                                                                                                                                                                                                                    ^

         line 462: Can't plot with an empty x range!

The problem occurs when replotting with title, xlabel, or ylabel commands.  The error message above is not an artifiact; the root locus in this case is indeed a pair of vertical lines that go outward from poles at -1 +/- j3.  It appears to be a problem interfacing to gnuplot, and that's where I get stuck.

Can someone duplicate this problem and, if so, how can I go about fixing it?

A. Scottedward Hodel, 334 844-1854, fax 334 844-1809

address@hiddenhttp://www.eng.auburn.edu/~hodelas