This tool is in early development - it will work best if you have all of the days data, morning and night. Without that it will not work hugely well. At a minimum this needs the buy price on your own island! (What you actually bought them for if it was on another island is not relevant)

I couldn't have done this without Ninji's work extracting the code

The first week in which you buy turnips from Daisy is treated specially - this tool doesn't handle that. It will only work if this is the second week onwards

To use - enter the buy price on your island, and any prices you have from the morning or afternoon of each day. The tool will figure out all possible patterns and predict the lowest and highest prices for each day.