This is a followup question of this one. It uses the same MWE but about a different issue so a separate it here. I used Tikz node to create the custom legend but used absolute positioning to place it where I wanted it to appear (below the plots, in the middle). I always try to use relative positioning when I create my Tikz images, but I don't know how to do it here. Or maybe there is some totally different approach in producing subplots with common legend?