Is this a useful metric, seem more like a trend following trade than a mean reversion spread trade? Have the fundamentals over time changed making the ratio less meaningful?
I think you mean gold/platinum spread you have to plot (gold - A* platinum) on the chart. Then determine if the trend is trending or reversing or trendless. A is a constant where value of gold futures contract = A*platinum futures contract The same concept applies to other spreads (eg corn/soy spread ....) You also have to ask yourself why you are comparing gold and platinum. For corn/soy spread, it could be meaningful as the farmer grows corn or soy.
My preference to use ratio charts is to see which seems to be stronger/weaker but not to pairs trade them or look for mean reversion.