Microeconomics,, 16th Canadian Edition

Every point on the price-consumption line corresponds to both a price
of the product and a quantity demanded; this is the information
required for a demand curve. In part (i), the consumer has a money
income of $4000 and alternatively faces prices of $1.50, $1.00, and $0.50
per litre of gasoline, choosing positions A, B, and C at each price. The
information for litres of gasoline demanded at each price is then plotted
in part (ii) to yield the consumer’s demand curve. The three points a
and c in part (ii) correspond to the points A, B, and C in part (i).