How can you request the journeys of 2017?
On the dashboard you can move to another year with a swipe movement.
If you swiped from left to right you get the dashboard of 2017.
How do I get an overview that can be used by the tax authorities?
From the dashboard you can do an export of your trips. The export is a csv file that you can import into Excel. This is exactly in the format you can use for the tax authorities.