The date picker (calendar widget) took me a while to figure out. When the period is set to “week”, “month” or “year”, there’s no obvious way to pick another week/month/year. At first I would (with the general period set to month) try to go to another month in the dropdown and close the date picker and wonder why the month that Piwik displays doesn’t change.
After a while I figured out that the date picker has a hidden state that is always a day. That hidden state is only indicated by a highlighted day number. It becomes completely invisible when you browse to another month. To actually select a new week/month/year, you have to click on a random day within that period.
The interface should allow me to directly select a period according to the general setting: When it’s week, select a week, etc.
It would also be nice to have a today/current week/month/year button.