Is there a tool to check citations over time across disciplines?
You may try Microsoft Academic for that. They have an interesting semantic-web approach to finding answers to your queries and they also aggregate a lot of context information. This includes citations that can be filtered in various ways.

Have a look at this search regarding your questions's example: https://academic.microsoft.com/paper/2067050450/citedby/search?q=Equilibrium%20points%20in%20n-person%20games&amp;qe=RId%3D2067050450&amp;f=&amp;eyl=Y%3C%3D1956&amp;orderBy=0 (you were right: initially, most citations were from mathematics).