The following image shows a graph of the yield of a crop as a function of the amount of fertilizer used:
Work Step by Step
The x-axis represents the amount of fertilizer used and the y-axis represents the yield of a crop. With no fertilizer (ie. when $x=0$), the graph already starts at some y-value, showing the yield of a crop without the use of fertilizer. As more fertilizer is used (ie. as $x$ increases), the crop yield increases as well (ie. $y$ increases). The relationship is not linear, however, as there are diminishing returns to using more and more fertilizer. Eventually, the crop yield no longer rises as quickly and levels off with increasing use of fertilizer.