Set your own switching rule
The Gold-to-Platinum Ratio
indicates how much platinum it takes to buy gold.
For example, if the prices of platinum and gold are 879.65 USD and 1,270.13 USD respectively, then the ratio is 1.4.
In other words, it requires 1.4 ounces of Platinum to buy 1 ounce of gold at these prices.
The Gold-to-Platinum Chart
indicates whether gold is historically undervalued or overvalued vs. platinum
When the ratio is low gold is cheap vs. platinum
When the ratio is high gold is expensive vs. platinum