The Pirc spike in the 1850s looks interesting visually, but keep in mind that your data set back then is incredibly small. Looking at my own database, the spike seems to be entirely due to some guy name Mahescandra playing it 57 times against Cochrane. It certainly doesn't have anything to do with the increasing popularity of 1.d4 40 years later.

I've never heard of Mahescandra, but Cochrane is the guy the famous Cochrane gambit in the Petroff is named after, where White sacrifices a piece on move 4 (1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nxf7).