This simple cylindrical projection is really a linear scaling of longitudes and latitudes (if you desire a different scaling for one of the axes you must choose the linear projection -Jx and append d for degrees; see Section 5.1.1.) It is also known as the Plate Carrée projection. All meridians and parallels are straight lines. The requirements to define this projection are:
A world map centered on the dateline using this projection can be obtained by running the command:
pscoast -Rg -JQ180/4.5i -B60f30g30 -Dc -A5000 -Gblack -P > GMT_equi_cyl.ps