What language is R written in?

The graph plotting lines of code vs. time is interesting. The lines for C and R steadily, almost linearly, grow over time. But the line for Fortran comprises long level periods punctuated by sudden jumps.<p>I assume that these sudden jumps represent existing Fortran libraries getting attached to the R system, and that organic growth of R is nearly all in R and C.

Note that an R line of code can potentially be equivalent to many lines of C code - more frequently than the opposite.

C