By the way, to really observe the difference, set one of your motors to go to a high speed, and you can HEAR it! It starts out accelerating very slowly... then part way through, it will take off (like in the non-linear chart I attached), and IF it doesn't stall, the last bit will accelerate so fast it's difficult to discern.
BTW, I graphed acceleration in the chart above intentionally... notice how the "Time Between Steps" chart is dropping... it is approaching 0. The graphs you attached start at 0, then accelerate away from it. That's because they were generated with "Steps per Second" in mind--your motor reaches max speed when it reaches your steps/sec target.
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