I'm not sure if I understood your first question well, but for the first question :
"Re the CPU Usage display: What is the unit of time for each unit of the x-axis?"
If you count the display ticks of that graph, you can see that there's one tick every second and that it goes through a full square in 5 seconds, so I guess 5 would be the answer if this is what you're asking.
For the second question :
"Re the PF display: What does "PF" mean" My reading shows a flat line of 1.21 GB; is that too high, too low? How do I modify it?"
PF stands for page file. It's a file where Windows store the data that cannot be held by your computer's RAM when it's filled up (full). Here's a good guide to understanding what the page file is and also how to modify it's values (with pictures included !) :
Does it answer your question ?