If you have a website that only came into existence from Dec 2016 but see traffic prior to that date, in the form of horizontal line shown in this example, read on for the explanation.
In December 2016 we expanded our database with a lot of new keywords.
This means that we crawled the Search Engine Results Pages (SERPs) for these new keywords for the first time and saw some websites ranking there and getting traffic for these keywords.
Due to the fact that we do not know if these sites were ranking for these keywords just "yesterday" or some time back, we had to make a decision:
Do we display this as if the website just ranked for this keyword, which would show a sudden spike of traffic?
Do we make the assumption that this website was ranking for this keyword for some time, and was already receiving traffic?
After some debate, we decided that the best option was #2.
We wanted to give the benefit of a doubt and assume these websites had been getting traffic for some time instead of showing an erroneous spike from 0 traffic.
Thus, websites that are only in existence around December 2016, will see a horizontal line in their organic traffic graph in the couple of months prior to that date.