Frequency Response Graph Reading
That's what they call a logarithmic scale

https://en.wikipedia.org/wiki/Logarithmic_scale

multiples have the same distance on a logarithmic scale 
eg: doubling: the distance 30 to 60 matches the distance
40 to 80 and 100 to 200 etc.

logarithmic scales are useful where arithmetic ratios are
more significant than arithmetic differences.