Sale by weight - quantity in stock

Hi,

It's not possible with variants.
It's possible if each variant is created as a different product.
In that case, you can have the 0.1kg product to be the one with the stock. If you have 10kg, you set the stock to 100.
Then, in the 0.2kg product, you add the 0.1kg product as a bundled product with a quantity of 2.
In the 0.3kg product, you add the 0.1 product as a bundled product with a quantity of 3.
Etc.
This way, the stock of the 0.1kg product will decrease automatically based on which product is sold and the quantity available for a product will depend on the stock of that 0.1kg product.
So for example, if you only have 10 in stock left for that 0.1kg product, when a user wants to buy the 0.3kg product, he will only be able to buy a maximum of 3.

You could potentially add the other variant products of the same product as "related products so that they would be easily accessibles on the product page.

If, one day, we add the capability to be able to setup bundled products in individual variants, then this mechanism would be possible with variants too, but not at the moment.