reshape the column vector to a matrix of the appropriate size, which is 8-by-225 (Ny-by-Nx). That can be done in two ways, and this way gives the more reasonable looking contour plot:
data = reshape(readmatrix('PKE_xy.txt'),[],8).'
% data = reshape(readmatrix('PKE_xy.txt'),8,[]) % the other way
Then make the contour:
contour(data) % x = 1:225 and y = 1:8 implied. use your x and y vectors here instead, i.e., contour(x,y,data)
