I absolutely love desmos. I’ve used it a little and decided to get serious today. Check out what I created.
Open the folder: Intervals, Area, and number of rectangles and move the n= slider for a quick look at what this can do. Then explore some more.
https://www.desmos.com/calculator/ydzdbw3pop
Here is a second one that goes along with it.
https://www.desmos.com/calculator/mimvmhpqx0