axis limits of hold on plot

[Evan]

for version 2.9.18, the default axis limits don't always include all
the data for hold on plots

for example

octave:1> version
ans = 2.9.18
octave:2>
octave:2> x=0:.1:2.*pi;
octave:3> y1=sin(x);
octave:4> y2=0.5*sin(x);
octave:5>
octave:5> plot(x,y1);
octave:6> hold on;
octave:7> plot(x,y2);
octave:8>

version 2.9.17 has no this issue, it seems due to recent changes.