A PDF PSA, or Never gonna set_xscale again -- guilty feats with
  logarithms
  In the course of doing astronomy, one often encounters plots of densities,
for example probability densities, flux densities, and mass functions. Quite
frequently the ordinate of these diagrams is plotted logarithmically to
accommodate a large dynamic range. In this situation, I argue that it is
critical to adjust the density appropriately, rather than simply setting the
x-scale to `log' in your favorite plotting code. I will demonstrate the basic
issue with a pedagogical example, then mention a few common plots where this
may arise, and finally some possible exceptions to the rule.
