mathythrowaway

But that does produce a picture identical to theirs. Because they are using 75 numbers per square, they do end up hitting at least one prime in each box with (n,k)=1 for the first 75 rows. This pattern won't continue much longer, which is why they've conveniently stopped the picture at that point.

No, it isn't plotting a prime spiral. Each square in their triangle is a block of 75 numbers. Within the nth row the numbers are sorted by residue mod n.

Their picture is correct, but only because they are doing something more ridiculous than this. Every one of the squares in their picture represents 75 numbers, not 5 numbers.

This just shows the reduced residues mod n. There are n squares in the nth row from the top. Color the kth square of the nth row black if gcd(k,n)=1, color it red otherwise.