Do you actually need plot lines, or are you trying to make the plot gridlines have a particular spacing?
If the latter, consider the example:
y=linspace(0,6*pi,100);
x=sin(y);
plot(x,y); grid on
yl=get(gca,'ylim');
set(gca,'ytick',(yl(1):pi/4:yl(2))-0.906)
Of course, that -0.906 offset obfuscates that the spacing is a nice pi/4
On the other hand, if you really just want a bunch of lines plotted, you can do that too.
clf
% pick how far you want the lines to extend in x and y
x=[1 10];
yl=[-2*pi 2*pi];
% calculate the array
y=repmat((yl(1):pi/4:yl(2))'-0.906,[1 2]);
plot(x,y,'b')