If you're using a logarithmic scale you shouldn't be using a bar chart, for the exact same reason as above - when you see bars, you naturally compare them on length. If the scale is logarithmic, then the form of the graph is in conflict with the data, and the graph becomes confusing. You should strive for every part of the graph to be in harmony and to repeat the same message.

The following paper on lucidity in scientific communication has been a big influence on my thoughts. Briefly, the author stresses that you should aim for lucid pattern repetition over gratuitious variation, i.e. the repetition of the same idea communicated in similar ways, over variety for the sake of appearing to be superficially interesting.

http://www.atm.damtp.cam.ac.uk/mcintyre/papers/LHCE/lucidity...