Thanks for the kind words everyone!
... You... you just made me notice that I uploaded a version where I flipped the canvas horizontally. Now all the metals are mirrored. I didn't notice because the composition is so symmetrical. *headdesk* Oh, well.
To answer your question: I just pulled up the table of allomantic metals and basically followed the order of metals there. So the four physical metals went in the top left corner, then the mental metals went in the top right, etc. I also considered having the feruchemical symbols in there too, but their spikey design clashed with the rest of the image quite badly.
Now, if you'll excuse me, I need to replace my uploads with a non-mirrored version.
I'd be honored! Make sure to get the non-mirrored version though