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

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.