When learning a new graph in economics, first ask yourself the purpose of the graph. Of course, household consumption depends on a great many factors besides income, e.g., household size and composition, individual expectations about future prices, and so on. Since these other determinants of consumption do not appear on the graph, the effects on consumption of changes in them are captured by shifts of the entire consumption line. If the change spells decreased consumption, the line shifts to the left. The latter reflect changes in consumption that are produced by changes in income itself.
Most economic behavior involves multiple behavioral relationships.
The prototype is the simple supply and demand model for an individual commodity in a competitive market. Of course, a and b represent different constants here from those of the consumption function. I reused the symbols, a and b, to emphasize that one must avoid memorization of symbols. For prices below P e , there is excess demand which will drive the price up. In this event both equilibrium price and quantity rise.