Circular convolution can be expedited by the FFT algorithm, so it is often used with an FIR filter to efficiently compute linear convolutions. These graphs illustrate how that is possible. Note that a larger FFT size (N) would prevent the overlap that causes graph #6 to not quite match all of #3.
<a href="//commons.wikimedia.org/wiki/User:Bob_K" title="User:Bob K">Bob K</a>
<span class="int-own-work" lang="en">Own work</span>