**Description of the desired feature** GMT's [grdhisteq](https://docs.generic-mapping-tools.org/dev/grdhisteq.html) would be a nice addition to PyGMT's grid processing capabilities. I am interested in submitting a PR wrapping pygmt.grdhisteq(). <!-- Please be as detailed as you can in your description. If possible, include an example of how you would like to use this feature (even better if it's a code example). --> **Are you willing to help implement and maintain this feature?** Yes <!-- Every feature we add is code that we will have to maintain and keep updated. This takes a lot of effort. If you are willing to be involved in the project and help maintain your feature, it will make it easier for us to accept it. -->