The plots would be more meaningful in logarithmic form.
For some purposes, maybe. However, the story here is simply whether/when the price goes up and down. For that a normal scale suffices. It's also more accessible to the average reader.
Source: I present data and charts to a variety of audiences for a living
why?
Because a change from $1 to $5 USD/btc is much larger than a change from $101 to $105 USD/btc. They needed two plots to show activity at two different orders of magnitude - that's the symptom. One plot with a logarithmic y axis would probably be a much better visualization.
Why is bitstamp consistently ~$10 cheaper per bitcoin compared to MTGox?
You can't withdraw USD from Mt. Gox, so people buy bitcoin to withdraw, driving the price up. Incidentally the same site did an analysis on that a few months ago as well: