[Note: The distance between cities in California (US) distance chart below is straight line distance (may be called as flying or air distance) between the two locations in California (US) calculated based on their latitudes and longitudes. This distance may be very much different from the actual travel distance.]
In drawing the demand schedule or the demand curve for a good we take income of the people as given and constant. When as a result of the rise in the income of the people, the demand increases, the whole of the demand curve shifts upward and vice versa. The greater income means the greater purchasing power.
A demand curve can shift to the left for a wide variety of reasons. A negative product review can reduce consumer demand for an item, shifting the demand curve to the left. Similarly, a government health warning about an item also can reduce demand and shift the curve.