My original example would suffice. Given a large number of small charts to be displayed, typically all of the same size, it seems quite reasonable to want them to be evenly distributed and to adapt automatically to use the available horizontal space, but for an incomplete final line to fill from the left under the items in rows above. (Please remember that this is what was meant by aligning to the left in this context; we’re not talking about literally aligning on the left margin without reference to the extra space added by flexbox here.) The alternatives available with the current flexbox behaviour tend to be things like putting a single chart in the centre or putting two charts on the far left and far right with a possibly huge space between them, which inevitably leave the items in the last line looking arbitrarily placed and disconnected from the rows above.

This is not much of an elaboration but the 5+4+4+4 I'm thinking along the lines of changing margin of nth children, or even specifically adjusting the CSS for the fifth child.

But how do you know in advance which n you need? The primary advantage of flexbox in this sort of situation is that your many items will use up the available display width while also being neatly spaced. If you know your exact horizontal sizes in advance, you don’t really need flexbox at all for this kind of layout, since again you can just choose suitable margins and/or padding on your items and then rely on simpler layout tools like inline blocks.