Add figure and example in cookbook for radians (#7585)

* Add figure and example in cookbook for radians

This PR adds another basic wouth-axis plot but here we demonstrate annotations in powers and fractions of pi.

* Update GMT_-B_radians.sh

Show an example of radians annotations

doc/rst/source/cookbook/options.rst

@@ -309,6 +309,26 @@ One solution is to request slanted annotations for the x-axis (e.g., Figure :ref
 via the **+a**\ *angle* modifier.
 
 
+.. _axis_radians_basemap:
+
+.. figure:: /_images/GMT_-B_radians.*
+   :width: 500 px
+   :align: center
+
+   Linear Cartesian projection axis with annotations and ticks in radians.
+
+.. toggle::
+
+   Here is the source script for the figure above:
+
+   .. literalinclude:: /_verbatim/GMT_-B_radians.txt
+
+There are occasions when the length of the annotations are such that placing them
+horizontally (which is the default) may lead to overprinting or too few annotations.
+One solution is to request slanted annotations for the x-axis (e.g., Figure :ref:`Axis label <axis_slanted_basemap>`)
+via the **+a**\ *angle* modifier.
+
+
 .. _axis_slanted_basemap:
 
 .. figure:: /_images/GMT_-B_slanted.*

doc/scripts/GMT_-B_radians.sh

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+gmt basemap -R-4/2/0/1 -Jx2.5c -Bxapi3fpi6 -BS --GMT_THEME=cookbook -view GMT_-B_radians