Nice work! You might be interested in these other libraries and tools that construct the call graph of a Go program:
golang.org/x/tools/go/callgraph
represents the call graph of a Go program
golang.org/x/tools/cmd/callgraph
computes the call graph of a program
golang.org/x/tools/cmd/godoc -analysis=pointer
displays the call graph of a Go program in a web interface