If you want to set/write a value, say 255, into some digital matrix (like an image) then you can do this:
function yourImage = drawpixel(yourImage, xp, y)
column = round(xp);
row = round(y);
yourImage(row, column, :) = 255;
Then to call that function in your loop, do this:
yourImage = drawpixel(yourImage, xp, y);
Note that images and line plots have the y axis direction flipped compared to one another, so if you want the plot on top of the image, you'll have to set ydir().