While we can't show income elasticity on a normal demand curve, we can graph desired quantity demanded on one axis and income on the other. Such curves are sometimes called Engel curves, or income demand curves. To the right is such a curve for an inferior good, when income is greater than I*.
Notice that even an inferior good must be a normal good at very low income levels. This makes perfect sense. After all,
one can't reduce consumption of a good that was never purchased,
so at very low income levels there are no inferior goods. Likewise, at very high income levels, almost anything can become
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