The output versus input graph of a linear system need not be a straight line through the origin. For example, consider a system described by y ( t ) = k d x ( t ) d t (such as a constant-capacitance capacitor or a constant-inductance inductor). It is linear because it satisfies the superposition principle. However, when the input is a sinusoid, the output is also a sinusoid, and so its output-input plot is an ellipse centered at the origin rather than a straight line passing through the origin.