In varnishhist the x-axis shows the time varnish took between getting the request from the kernel and sending it back to kernel.
The y-axis shows the number of requests.
| => cache hit # => cache miss
That is what I understood from the manpage. Correct me if I'm wrong.
Yet there is one thing I'm unsure about. The pipes (|) on the far left side, do they also stand for cache hits or are they just there to print the y-axis? I'm voting for the latter, but would like to be sure.