jQuery Grid Plugin - jqGrid

I am having some problems sorting formatted data.

I have a column with the underlying values -1,0,1, but I want to display as '-', ' ', '+'. I also need these to sort as they would using the underlying integer values. I am using local data and the TableToGrid plugin.

I tried using creating the table using -1,0,1, and using a custom formatter. This displayed the values correctly, but I couldn't make it sort on the underlying integer data, only as text using the +,- symbols, so the sort order is displayed wrongly.
Problem 1: I think this is a bug and sorting should be done before formatting is applied

To find a workaround I saw in /jqgridwiki/doku.php?id=wiki:colmodel_options that there is a custom sorttype option. However I cannot find any documentation for how to use it, either in the examples or the documentation.
Problem 2: can someone show me how to use a custom function for sorttype instead of sorttype: int, float etc.

Thanks,