Illustration of <a href="https://en.wikipedia.org/wiki/Monotonic_function" class="extiw" title="en:Monotonic function">non-monotonic function</a>
<a href="//commons.wikimedia.org/wiki/User:Oleg_Alexandrov" title="User:Oleg Alexandrov">Oleg Alexandrov</a>.  Stroke converted to paths by <a href="//commons.wikimedia.org/wiki/User:Superm401" class="mw-redirect" title="User:Superm401">Superm401</a>.
Made by <a href="//commons.wikimedia.org/wiki/User:Oleg_Alexandrov" title="User:Oleg Alexandrov">Oleg Alexandrov</a> with <a href="https://en.wikipedia.org/wiki/Inkscape" class="extiw" title="en:Inkscape">en:Inkscape</a>.  SVG code below from <a href="//commons.wikimedia.org/wiki/File:Monotonicity_example3.png" title="File:Monotonicity example3.png">Image:Monotonicity_example3.png</a>; function's stroke converted to path for uploaded version due to rsvg rendering bug.