What does the x-axis represent on the graphs in the doctor app (https://mcollina.github.io/.... Is it the event number - therefore each sample is taken at every new event loop tick?

That are realtime seconds, basically the time at which the sample was collected.
That visualization was taken with quite an old version, you can see a much recent one at https://upload.clinicjs.org... and https://clinicjs.org/.

Do you have a demo for memory leak diagnosis?

memory leaks tend to cause GC issues - there's an slow gc example at https://github.com/nearform... you can start that server with `clinic doctor` and then load test the server with apache bench, wrk2, or autocannon (http://npm.im/autocannon)