Does price affect quantity or vice versa? On nearly all graphs found within economics textbooks, quantity is label on the x-axis and price on the y-axis, implying that the quantity supplied affects the price. Is this really so? Intuition would tell you (or me at least!) that the quantity someone wishes to buy is dictated by the price, which in turn is decided by whoever is selling the item.
This is a common problem among students first learning economics. The way these graphs are displayed are not meant to tell you anything about causality, let along the direction of causality. The reason that price is on the y-axis is partly because of tradition and partly because that's what makes the most sense from the perspective of developing economic theory. 

One of the central questions of economics is the determination of prices. What causes prices to be what they are? So, it maybe seems natural to write relationships that give a price as the dependent variable. However, the layout is not meant to convey any information about causality.