def _generate_items(self, df, columns):
"""Produce list of unique tuples that identify each item."""
if not self.bin:
super(ColorAttr, self)._generate_items(df, columns)
else:
if len(columns) == 1 and ChartDataSource.is_number(df[columns[0]]):
self.bins = Bins(source=ColumnDataSource(df), column=columns[0],
bin_count=len(self.iterable), aggregate=False)
if self.sort:
self.bins.sort(ascending=self.ascending)
self.items = [bin.label[0] for bin in self.bins]
else:
raise ValueError('Binned colors can only be created for one column of \
numerical data.')
