Panel a): comparison of the N-S (red) and E-W (blue) variations of the granulation contrast: individual selected images (dots); 8th order polynomial fits, computed separately in the N, S, E and W hemispheres (full lines). The error bars are weighted standard deviations of the individual values with respect to the polynomial fits, computed along the polynomials. They are only plotted in a few positions on the N-S polynomials, but they are as short on the E-W polynomials. The error bars are very short, because a large number of individual values are involved in the computation and their dispersion is rather small. Panel b): difference between the N-S and E-W variations, relative to the granulation contrast at the same distance from the disk center. The errors along the difference between the N-S and E-W polynomials are the sum of the N-S and E-W errors.