How is the Y value for the Memory Pressure graph in Activity Monitor calculated?
This is currently only a partial answer. This answer not complete until this sentence removed.

The thing about this question is that on the face of it, the Memory Pressure graph doesn't quite make sense when comparing it with what's reported in Terminal when using the command outlined by the OP:

sysctl -a vm | grep page_free &amp;&amp; sysctl vm.memory_pressure &amp;&amp; sysctl kern.memorystatus_vm_pressure_level


However, the problem here is that this command limits the vm values it's reporting back. In fact, there's actually 105 values in total but this command only reports the following five values:

vm.vm_page_free_target:
vm.page_free_wanted:
vm.page_free_count:
vm.memory_pressure:
kern.memorystatus_vm_pressure_level:


NOTE: Numerical values would normally appear after the colon (:), however I have not included these.

Obviously, the overwhelming majority of the 105 vm values are not part of the actual calculation that determines memory pressure, but the problem here is that the five values above are not the only ones used to calculate memory pressure. 

For example, while the above output provides some of the various free values, it's not providing any of the inactive values, etc. 

These values are important because the memory pressure algorithm calculates a ratio between those values, and the relationship between those values isn't apparent from the output produced by the command in the OP's question.

I think I may have some technical information that can shed more light on this. I will update this answer either way within the week.

[STATUS UPDATE]

I've been unable to find the technical information I was looking for. However, I am fairly certain I do still have it - probably on some old CDR/DVDR backups. I will update this answer once I've found it, or after I'm certain I won't find it.