ieeetr bibliography style is one of the original BibTeX styles. It dates back to (at least) the early 1990s. As such, maybe unsurprisingly, it's not programmed to do anything with a
I suggest you use the
IEEEtranN bibliography style instead. It is much newer and does know how to process the
url field. It also doesn't ignore the
address field. If you follow this approach, you should load the
natbib package with the option
numbers. In addition, you may want to drop the string "Online" from the
note field, since the string "[Online]. Available:" is prefixed to the
url field by the IEEEtranN style.
A separate remark: I'm afraid I don't understand a single word of Finnish, but I suspect that one or more of the words
Sähkön should not be converted to lowercase by BibTeX. If this hunch is correct, you should encase the words in curly braces, to exempt them from the "sentence style" conversion that's applied by the IEEEtranN bibliography style to the contents of
author = "Tilastokeskus",
title = "4. Vuosineljännes 2014, Liitekuvio 12. Sähkön tuonti ja vienti 1990–2014*",
address = "Helsinki",
url = "http://www.stat.fi/til/ehk/2014/04/ehk_2014_04_2015-03-23_kuv_012_fi.html",
note = "visited 6-May-2015"