[Changeset] plotyy leaves traces of previous plots

[John W. Eaton]

On 31-Mar-2008, Ben Abbott wrote:

| The manner in which plotyy responds is very strange to me ... a
| bug/feature I think ;-)
| 
| I'd prefer if Octave's version were to not copy this
| behavior. Rather the second plotyy should place (x3,y3) in the 1st
| axis and (x4,y4) in the second, with all four lines visible.

I don't think we get to choose if we want compatibility.  Although the
plots might appear to be the same, code written for Matlab that uses
the plot properties would break and users would likely report the
difference as a bug.

jwe