after some searching: it seems like you can have a linear quantity in physics or in dimensions when it involves length. for example, the more fence you have, the more length it has. it's a linear quantity because it scales 1:1
outside of that you can have a linear relationship, which is more like: income per hour worked. the more hours you work, the more income you have. and ignoring overtime pay, it's a 1:1 ratio again
i dont believe the graph involves a linear relationship, because the the filesize doesnt increase the same every year. there's no connection between time progressing meaning filesize is inherently bigger. and file size isn't linear quantity because it doesnt involve a length.