You cannot pick a single commodity and compare against the USD, mainly because traders flip between all commodities, not just one. It has to be a basket. But it certainly correlates as Willrocks shows.
To answer the OP question, what is keeping it at the current level is the technical support level close to where it is, and another around $12.45-$12.50 (that rough $400/kg mark). If it breaks $12.40 then it''s going lower to your kind of level.