matplotlib-0.98.5.2.ebuild contains "excel? ( dev-python/xlwt )" in $RDEPEND but "excel" is not in $IUSE. A grep of the sources shows that lib/mpl_toolkits/exceltools.py uses xlwt so I think that the "excel" flag should be added back.
Thanks for reporting this issue, assigning to maintainers.
fixed, thanks.