Seasonality visualizations: Day-of-the-year

[JamesKunstle]

Greg Sutcliffe's visualization idea in #204:

"Day-of-year gives maintainers some way to adjust expectation for the coming time-horizon (i.e is it March? I might expect a dip in contribution then, and should not panic when it happens). Clearly this does not work for new projects (STL requires 2 time periods, so 2 years for this)"

Refer to figure suggestions in #203, and consider creating as a second-axis of #203.