Self-paced learning: location of /graph/overlay2 dir on ubuntu

I’m working through the docker self-paced learning and I’m not able to figure out where the /graph/overlay2 dir is… here is the link to the training page where its referenced. (one of the bottom of the page commands, ‘ls /graph/overlay2/’) Any help would be great. Thanks!

Docker version: Docker version 17.06.0-ce, build 02c1d87

Here is a peek at my /var/lib/docker dir:

root@drewbuntu:/var/lib/docker# sudo tree -L 2 ./
./
├── aufs
│ ├── diff
│ ├── layers
│ └── mnt
├── containers
├── image
│ └── aufs
├── network
│ └── files
├── plugins
│ ├── storage
│ └── tmp
├── swarm
├── tmp
├── trust
└── volumes
├── 2cff3a2695d1ca579699736001a289087bc184f40347c958389cfa0a8c6ea536
├── 6dcc85dcee216c2a97376677adf18b2dce76012ed3ffc040bf1683e9ac5c5b17
├── 703d1192462e0a0bf9c52b1233ea517c57007b10138753653ed1c9ea67139b3c
├── 9ac1b783d5fb735c750971843b77de595ca393bbe4b8aa592987b7658e382101
├── dc75918752331905fd4ecaabf838e475d3fe18fb4fcc434d130bc498812f6653
├── docker4wordpress_codebase
└── metadata.db

try this instead: ls /var/lib/docker/overlay2