Scales in tmap are configured by the family of functions with prefix tm_scale.
Such function should be used for the input of the .scale arguments in the
layer functions (e.g. fill.scale in tm_polygons()). The function tm_scale()
is a scale that is set automatically given by the data type (factor, numeric, and integer)
and the visual variable. The tmap option scales.var contains information
which scale is applied when.