[scikit-learn] Is there rule to determine X and y when train regression?

[lampahome]

I found many example to predict stock, house prices, taxi fare...etc.

The field of y almost like below:
y : the price of the day
And X maybe the day, param which can affect price...etc.

Now I want to predict sales of multiple items of multiple stores.

Is suitable to let decrease/increase ratio of sales be y?

The reason I'm interesting is I don't know how to explain to other people
that price as y is normal.
So other people may have a question that can we let y be increase/decrease
ratio?

thx
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