Add "use strict" to graph.js

to solve eslint problems

```
lib/graph.js
   27:17  error  Missing "use strict" statement  strict
   63:15  error  Missing "use strict" statement  strict
   64:16  error  Missing "use strict" statement  strict
   87:22  error  Missing "use strict" statement  strict
   97:22  error  Missing "use strict" statement  strict
  263:22  error  Missing "use strict" statement  strict
  270:28  error  Missing "use strict" statement  strict

✖ 7 problems
```