How to plot my u-solution along y coordinate at constant x coordinate?

I want to plot my u solution of pde nonlinear along y coordinate at constant x coordinate. The plot will be along the line of [xmax/2,ymin] [xmax/2,ymax]. I did not understand the nonlinear heat transfer example plotAlongY(p,u,0);, what does the "0" means inside the bracket?

I tried the answer from Bill Greene (below), but I got error. plotAlongLine(p, u, [0,.5], [1,.5], 25); function plotAlongLine(p, u, xy1, xy2, numpts) x = linspace(xy1(1),xy2(1),numpts); y = linspace(xy1(2),xy2(2),numpts); F = TriScatteredInterp(p(1,:)', p(2,:)', u); uxy = F(x,y); figure; plot(x, uxy); end

Please help me. Thank you.