Thanks for org-habit, it is a very nice feature!

When the last DONE date is earlier than `org-habit-preceding-days' the
graph can not be rendered due to error in org-habit-build-graph.

I have tracked down the problem (calling time-less-p on nil when
done-dates is empty) and the following patch is solving the issue for me
but I am not sure if this is how you intended it.