The length of the bar is linear with the memory up to 512K. For the next
1024K the gradient is a half, the next 2048K a quarter and so on (for very
large values this approximates to a sum of logorithms)

Importantly
- its fast BOTH ways (the inverse is required for dragging)
- linear for low memory sizes- easy to use
- total memory/ used/free etc. aren't much bigger

