Freeing up space · Storage Profile, charts and Home
Why does the storage chart have two different scales?
Short answer: Your built-in and fast drives are often very different sizes, so each gets its own colour and scale; otherwise the smaller drive's changes would be flattened.
So both drives read clearly. Your built-in drive and your fast drive often differ a lot in size — say a small internal drive and a much larger external one — and on a single scale the smaller one would be flattened into a line along the bottom.
Each drive gets its own colour and its own scale, so changes on either are easy to see. Hover a point to read the exact value. See What do the storage charts show?