How to build an approximate server load graph using sys_getloadavg?

Question

PHP

I know how to plot graphs.
How to get at least an approximate average value?
Please tell me the formula, otherwise the function
sys_getloadavg
gives me

array(3) { [0]=> float(1.01) [1]=> float(0.94) [2]=> float(1.15) }

I read the documentation, but I did not find anything normal.
Please help me with 2 questions:
1. Write a formula for calculating the approximate average value (accuracy is not important)?
2. Will this sys_getloadavg function cause unnecessary load on the server?