Art Appreciation: A Post Peak Phenomenon

This graph shows the ascending trend in the prices of fine art paintings sold at auctions from 1984 to 2006. I chose this graph in part because this topic fascinates me, but also because when I look more closely at the graph, I realize that there are two things that remain unclear to me. It seems as though the y-axis measures price, but I don't see how the numbers plotted on the actual graph correlate to the labels on the y-axis bins. I'm also unsure of how a "monthly, log scale" is incorporated into this graph whose time is measured in years, not months.