A rhumb line at 45 degrees to meridians is transferred from sphere to the the plane chart (equirectangular projection). The parallels are adjusted upwards to give a straight line as for the Mercator projection.
<a href="//commons.wikimedia.org/wiki/User:Peter_Mercator" title="User:Peter Mercator">Peter Mercator</a>
<span class="int-own-work" lang="en">Own work</span>