How to counter Janna
Let us take a look on how to beat Janna. We will look at champions who are strong against Janna and who are weak against Janna. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Support champion who counters Janna. You can already win in champion select with the right counters for Janna. If you want to know how to counter Janna then you need to take into account the best picks vs Janna and the worst picks vs Janna. This Janna counter data is based on matches played by Pros on patch 13.24 and previous ones. It is important to see whether Pros counter picked Janna or whether they played as Janna.
Champions used by Pros to counter pick Janna
Looking at the matches from Pros (see table Pros counter picked Janna) we can see that they picked 
Senna most often to counter pick Janna. The matchup Senna vs Janna occured 32 times. Janna has 20 wins and 12 losses in this matchup. Learn how to counter pick Senna to get another perspective on the data.
Lane outcome for Janna
When looking at Janna counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Janna. The lane outcome of Janna consists of three outcomes: Win, Loss or Draw. Let's take a look of Janna lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Janna.
Conclusion
Analyze the available data and learn how to beat Janna. To master Janna take also a look on Janna builds and runes for Janna. Good luck, Summoner!