plot style changes when printed to files

[Evan]

version 2.9.15

for example,

octave:1> x=0:.1:2*pi;
octave:2> y1=sin(x);
octave:3> y2=cos(x);
octave:4> plot(x,y1,"-",x,y2,"-");
octave:5> print -deps a.eps
octave:6>

the plot displayed on screen is line style, however, in a.eps, one of
the lines changes into dashed line.

how can I keep the line plot style not changed when saved to files?

thanks in advance