"can we plot a sine wave which half wave i.e. positive side only"
Yes. Here's a hint. This produces one full cycle of a sin curve between -pi and pi. You'll want to plot it from 0 to pi.
x = -pi : 0.01 : pi;
y = sin(x);
plot(x,y)
grid on
"can we plot same sine wave in reverse direction from above plot "
I'm not sure what this means. Perhaps you want the section between -pi and 0?
" how can i plot a cosine wave which has a centre on origin "
Hint: use cos() instead of sin().
