no "end" date on the graph should mean "today"
On the metrics graphs, when you can choose dates, if I choose a start date but no end date, it ignores my start date. It should instead fill in "today" for the end date.
(We might also want it to fill in "the beginning of my data set" for start date if an end date is specified but no start date, but that behavior is a bit more surprising, so I could go either way there.)