In Canada I think they are legally required to, but sometimes it can be frustrating if they don’t always compare like units - one product will be price per gram or 100 grams, and another price per kg. I’ve found with online shopping, the unit prices don’t take into account discounts and sale prices, which makes it harder to shop sales (in store seems to be better for this).

I doubt it. Seems totally optional here where I am in BC.

What they often do is put different units on the same type of good. Three chocolate bars? One will be in oz, one in lbs, one in "per unit."

They all are labelled, but it's still customer hostile to create comparison fatigue.

This is such a shame, anywhere this is mandated they should mandate by mass and for medical/vitamins per mass of active ingredient

Worse, I've seen CVS do things like place a 180-count package of generic medication next to an identically-sized 200-count package of the equivalent name brand, with the generic costing a bit less, but with a slightly higher unit price due to the mismatched quantities.

I live in BC, common to not see unit pricing.