What type of Y-axis scale is most often used for graphs of circuit frequency response?
Why Circuit responses can span enormous ranges, from unity gain down to a millionth of the input, so a linear vertical scale would squash all the small values against the axis. Plotting gain in decibels, which is a logarithmic measure (dB = 20 log(Vout/Vin) for voltage), compresses that range into a readable graph and turns rolloffs into straight lines with slopes like 6 dB per octave. This is the standard Bode plot format, and the horizontal frequency axis is usually logarithmic too.
Watch out A linear scale is fine for a narrow range of values, but it hides the tens of dB of attenuation in a filter stopband that engineers most want to see.
Frequency response is plotted in dB, and dB is a log unit, so the axis is logarithmic.