Add margin for footer

Prior to this commit, there was no margin at the bottom of the page
and the footer wold cover up the bottom of the last two graphs. Now
there is a margin of 75px at the bottom of the final graphs to ensure
they're not covered by the footer.
Update default time span to match slider

Prior to this commit, the slider value defaulted to "last year" but
the graphs would default to "all time". Now they both default to "last
year" so that everything is consistent.