But if the current displayed maximum of network I/O is at 3.28 MB, the grid lines at 25% (0.82 MB), 50% (1.64 MB) and 75% (2.46 MB) are just as odd as before. No improvement.
I'm not sure if it's because I'm fine with mental arithmetic, but this seems like a very reasonable thing to do. From the maximum you can easily work out a ballpark figure for other values on the graph.
Then, the current scaling value is displayed just where the maximum value is shown in the graph. It's only a single value. So you know what value was current at that single point. But due to the lack of a usable grid, other values are still visual guesswork.
You want each horizontal line to have an associated label?
I'd recommend you to open an Excel document, insert some data, and create a basic line graph. The default settings should give you an idea of what is usable so you can compare it with what you have now. It's not there yet.
Why are you comparing the PH graphs to Excel graphs? The emphasis for PH is to have as little clutter as possible, and to make big spikes easily visible. I don't want the sys info window looking like a massive spreadsheet with numbers everywhere. Why do you care about what the precise figures are at some specific point in time?
Also take a look at Windows 10's system task manager. It does the graph scaling pretty well. I also like how the visible time span is shown there. This has improved a lot.
Hover over the graph and look at the precise time there.