'Converts a discrete aesthetic to a value that will be displayed. For example
from "a" => "#4682B4".'

def _get_mapping(self, aes_type, colname):

    mapping = None
    if (aes_type == u'color'):
        mapping = discretemappers.color_gen(self.data[colname].nunique(), colors=self.manual_color_list)
    elif (aes_type == u'fill'):
        mapping = discretemappers.color_gen(self.data[colname].nunique(), colors=self.manual_fill_list)
    elif (aes_type == u'shape'):
        mapping = discretemappers.shape_gen()
    elif (aes_type == u'linetype'):
        mapping = discretemappers.linetype_gen()
    elif (aes_type == u'size'):
        mapping = discretemappers.size_gen(self.data[colname].unique())
    elif (aes_type == u'group'):
        mapping = discretemappers.identity_gen(self.data[colname].unique())
    return mapping
