__proxy__ found error if translated field supplied as "legend_by"

[durdenk]

django-chartit/chartit/charts.py

Line 607 in 0934cac

lgby_titles = (':'.join(lgby_vname_list).title() for

This line causing "sequence item 0: expected str instance, proxy found" error if a translated field is found.```

        lgby_titles = []
        for lgby_vname_list in lgby_vname_lists:
            tmp_list = (str(vname).title() for vname in lgby_vname_list)
            lgby_titles.append(':'.join(tmp_list))

if wrote like above, no errors. Its identical, but this list yields proxy fields for translated fields if wrote as original.