GRASS Basics - Desktop GIS: Mapping the Planet with Open Source Tools - page 307

Figure C.8: Settings for a thematic map of the world by population
the population field is precisely that. If you can't remember the name
of the fields in the layer, GRASS lets you list those using the Show
Attribute Columns button. Clicking it causes the field names and types
to be printed in the Output window. Doing that we quickly see that
POP_CNTRY is what we want to use to classify the map so it gets entered
in the textbox for the numeric attribute column.
Next we need to set the type of thematic map from the drop-down list.
In the case of the population map, we want graduated colors. You can
also select the Map By option, choosing interval, standard deviation,
quartiles, or custom_breaks. Interval will produce the results we need.
We set the number of intervals (classes) to 5 and choose a preset color
scheme of Green-Red. That's all we have to do to set up the map. In
Figure C.8 , you can see the completed setup in the GIS Manager for
the thematic map. There are more options you can't see in the figure,