One way to title your graph is to follow the "y-axis variable vs. x-axis variable" format.
Example: if you were comparing the the amount of fertilizer to how much a plant grew, the amount of fertilizer would be the independent, or x-axis variable and the growth would be the dependent, or y-axis variable. Therefore, your title would be "Amount of Fertilizer vs. Plant Growth."
You can also title your graph by describing very briefly what the graph is showing. This method still requires you to state the y-axis variable and the x-axis variable.
Example: "The Effect of the Amount of Fertilizer on Plant Growth".