Plot middle of two lines (Hough Transform)

My program is detecting the desired lines

But how do i plot the middle of them lines.

Program is plotting lines (green), start point (yellow), end point (red)

% Hough Transform 
[H,theta,rho] = hough(filled_a_image);
P = houghpeaks(H,2,'threshold',ceil(0.3*max(H(:))));
lines = houghlines(filled_a_image,theta,rho,P,'FillGap',5,'MinLength',10);
figure, imshow(filled_a_image), hold on
max_len = 0;
for k = 1:length(lines)
   xy = [lines(k).point1; lines(k).point2];
   plot(xy(:,1),xy(:,2),'LineWidth',1,'Color','green');
   % Plot beginnings and ends of lines
   plot(xy(1,1),xy(1,2),'x','LineWidth',2,'Color','yellow');
   plot(xy(2,1),xy(2,2),'x','LineWidth',2,'Color','red');