use archive.org, it provides https..

NOTES.md

@@ -1,3 +1,3 @@
 Documentation for graph-easy, which is used to convert diag_inp.txt to
 the outputs, diag.*:
-http://bloodgate.com/perl/graph/manual/index.html
+https://web.archive.org/web/20200120140454/http://bloodgate.com/perl/graph/manual/index.html