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First of all: thanks for the upgrade! Very helpful!!!

Is it possible to set the start and the end of the fit line. For example analyzing the motion on an inclined plane (roll experiment e.g.) using only the date of the acceleration phase?
No, this function just does a simple fit to the data.

However, when in data picking mode, you can drag between two points instead of picking a single point. This gives you the differences and the slope between the points. We found that picking a start and end point. We found this to be a more intuitive and instructive approach than supplying a start and end point to a mathematical fitting function.

Does this help your needs?
About the Linear fit, I missed the errors (uncertainty) on a and b values. I think that defining a start and stop to linear fit will be a great feature. How about using only the data shown in the zoom?
The uncertainties should be no problem. I am not sure if many users are able to interpret them correctly (there are so many ways to misinterpret these even for a linear fit), but I will add them.

(This reminds me of students who did a linear fit on temperatures in the range of 10°C to 80°C to find at which point they cross 0°C. The results were quite good, but they got an error of +/-1000°C because they did their calculations in Kelvin. Understanding why the error was much smaller when doing the same math in °C took a while...)

Selecting the range is a bit more difficult. If we just use the zoom, should the fit be updated when the user tries to zoom into the fit results? Currently the fit also updates as new data arrives. This is very useful for the centripetal acceleration experiment, but how should this be handled when the zoom follows new data. I think the range should be independent of the zoom, but I am afraid that this is a bigger task which we should should tackle in a later update. The tool-menu will certainly grow in the future...
My ideas:
At the moment I am using the app GraphicalGW (from Vernier) to analyze data from Phyphox. You can export data from Phyphox to GraphicalGW.
Selecting the range and getting a fit is very easy in this app. Maybe you can find there some ideas.

But in Phyphox I find it better, that you can switch between the "zoom-mode" and the "data-picking-mode", because the zoom is fix, when you analyze the data.

To the linear fit:
For school (I think) it would be easier, when you can select a range and get a linear fit for this selection instead of selecting two points and getting the slope.

Thanks for the great app!