If you want to read it as one would normally, where the X axis is the input variable, you could read it as "For a given quantity of demand, what is the maximum price the market can sustain?" So if you want to sell higher quantities of oil, the price has to decrease. The supply side is the opposite direction since costs go up as quantities go up (ignoring efficiencies of scale).

Since the graph is without units, the only relevant of their positions are the signs of the slopes, and that you need a higher price to supply oil at any given quantity (hence the Straight closed" line being higher on the graph).