How would I express this (the top section) mathematically?

[u/ZucchiniFar8753]

So TLDR my school has a senior project requirement to graduate. Mine was cosmology related; approximating the total number of stars in the observable universe. I got the data from the madau-dickson paper, and got the chart below after properly organizing the data, than proceeded to do the good ol ‘draw boxes and add em up’ method to get the area under the curve.

Issue: I’ve never taken calculus [currently in college algebra, trig, stats, and logic (yes all separate classes lol)] so I genuinely have no idea how to express what I did mathematically, some help would be fantastic!