Intensity-wavelength graph for emission spectra; why is frequency in the x-axis?

[slakedlime]

In my astrophysics book (Astrophysics, Nigel Ingham), there are intensity-wavelength graphs for line spectra (both emission & absorption). On the y-axis, relative intensity is plotted. But on the x-axis, frequency and not wavelength is plotted. Why is this? If I wanted to convert this graph to an actual intensity-wavelength graph (e.g. like one for a black body), what would I have to do?

 

[astrorob]

Just remember how frequency and wavelength are linked... The choice is relatively arbitrary.

 

[cepheid]

I like frequency better. It has the advantage that it scales linearly with energy, so that you know that the rightmost end of the graph corresponds to high energies, and the leftmost end to low energies.

I'm not sure why you seem to think that plots of the Planck function (blackbody radiation) are always in terms of wavelength. The Planck function can be expressed in either form (as a function of either lambda or nu).