Panel a): comparison of the granulation scale variation along the central meridian averaged between the northern and southern hemispheres (red line), with the variation along the equator, averaged between the eastern and western hemispheres (blue line). Panel b): difference between the N-S and E-W averages relative to the granulation scale at the same distance from the disk center; the error bars are derived from the dispersion of the individual values (see Sect. 4).