It can be useful in exploratory graphics to impute data outside the range of
the data. impute_below imputes all variables with missings to have values
10 percent below the range for numeric values, and for character or factor
values adds a new string or label. It is powered by shadow_shift, so please
see the documentation for shadow_shift() to full details on the different
implementations.
impute_below(...)extra arguments to pass - see shadow_shift() for discussion on this.