Log-Log Plot of Electromagnetic Radiation:

In short, the type of coordinate system used to plot data, cartesian, semi-log, or log-log, should be based on the kind of function to be graphed and the desired shape (curve or line) of the curve wanted.

Cartesian system - Linear (y = mx + b) category functions while plotted will provide straight lines; exponential functions (y = ex) will plot as curves.

Semi-log system - Should not plot linear category functions on semi-log.  Exponential functions, like as radioactive decay and reactor power equations while plotted will graph as straight lines.

Log-log -  Rarely used to plot power equations.

Figure: Log-Log Plot of Frequency vs. Wavelength of Electromagnetic Radiation