That does exist but is in beta form. You can long-press on the scales and you'll get a dialog to set the range.
I agree it is a basic functionality and easy to implement but the difficulty is in the design. I'm not sure the current design is right and if I change the design later, it causes angst for users, support emails and sad review. I have learnt it's harder to remove a feature compare to adding it.
Maybe this is the right time to discuss it.
Design 0: the current design is that you set the data range for each data for each place. This gives full flexibility but can be cumbersome is you want to set the wave height range for 10 places.
Design 1: you create a global range setting, just like graphs, then you can set the data for each place to a global range setting. We can have a preset default ranges for each data type so you don't have to assign a range to each data type.
Design 2: there are preset global default ranges for each data type which you can customise. But you cannot change the range assigned to a place data. I don't think this will work because you may want different temperature ranges for different places.
Design ?: some combination of the above.
It is important to get this design right because to change a design (database schema) and migrate data is damn hard and prone to major bugs. I have learnt this the hard way with 4 redesigns of data downloading and 2 database redesigns with a third coming soon.
I hope you see Flowx, not as lacking basic functionality, but a series of carefully considered designs :-)
On the note of beta ranges, expect there to be bugs and for the ranges to be lost when I implement the final version.