If you talk of a scatter plot, regularly the x-axis is used to plot the explanatory variable, this is the possible cause; while the y-axis is used to plot the varibale that is being explained, the result, the efffect.
If you are dealing with a function or model, you the convention is to use the x-axis for the independent variable, this is the input, while the y-axis is for the output or dependent varibale (which is determined by the independent variable).
