Why are there sometimes 0 visits, but some conversions recorded, for example for a Keyword, a Website or a Custom Variable?
Let’s go through the imagined scenario:
1. User comes to your website via the campaign “Mailing January”, with a click on a link in an email tagged with Piwik PRO (or Google Analytics) campaign parameters.
2. He looks around and exits after 3 pages
3. He comes back the next day (on 2018-01-01), through a direct entry, and buys a product on your site (which is being tracked as Goal Conversion)
In this case, if you select the date 2018-01-01 in Piwik PRO, it will show that the “Mailing January” Campaign did not have any visit on this day (because the user entered the website directly), but the conversion is attributed to the Last known referrer that the visitor used (it is stored in the first party cookie). On 2017-12-31, there will be one visit from the campaign “Mailing January” but no conversion on this day.