The slope of the demand curve is only influenced by changes in price of the commodity in question(x). In fig 1 and 2 above, the demand schedule provides changes in quantity demanded in response to changes in price.  From fig. 1, we notice that the lower the price the greater the number of goods purchased.  When we plot these combinations we get the demand curve in fig. 2. This results in a movement along the demand curve referred to as change in quantity demanded.

b) Deriving a Market Demand Curve
The market demand curve is a horizontal addition of the individual demand curves as shown in fig. 3 below. For example, when the price is 6 shillings, consumer one (left) purchases 30 units while consumer two (middle) purchases 60 units.  The total market (right) demand is 90 units.