In this financial market the vertical axis shows the interest rate, the price. Demanders are households and businesses; suppliers are the companies that issue credit cards. The figure uses no specific numbers, which would be hypothetical anyway, and focuses on the underlying relationships.
Imagine a law imposes a price ceiling that holds the interest rate at Rc, below the rate R₀ that would otherwise prevail. The horizontal dashed line at Rc is the ceiling. At the lower rate the quantity of credit card debt demanded rises from Q₀ to Qd, but the quantity supplied falls from Q₀ to Qs. Quantity demanded exceeds quantity supplied, so a number of people who want cards and would pay the going rate find that companies will not issue them one. The result is a credit shortage.
Many states have usury laws that cap interest rates, but in most cases the cap is well above the market rate. Raise the ceiling above R₀ and it stops binding: it has no practical effect unless the equilibrium rate rises to meet it.