how to create a line through a point and make it turn

I have found the centroid of a picture with the following code:

Q=gpuArray(imread('ISIC_0000001_segmentation.png'));
S=regionprops(Q,'centroid');
%plot centroid on picture
centroid = cat(1, S.Centroid);
imshow(Q)
hold on
plot(centroid(:,1), centroid(:,2), 'b*')
hold off

Now I want to find the line of symmetry of that picture that goes through this point, but im not sure how to do it. I thought of creating a line that goes through the centroid, and having matlab calculate the % of symmetry with that line, and then change the tangent of that line and calculate the symmetry and so on and finally take the lowest percentage. Any ideas on how to do this would be great!

[KSSV]

Through a point..you will get infinite lines......you need two points to define a line. Attach your image and tell us your expectations.