How to counter Yuumi
Let us take a look on how to beat Yuumi. We will look at champions who are strong against Yuumi and who are weak against Yuumi. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Support champion who counters Yuumi. You can already win in champion select with the right counters for Yuumi. If you want to know how to counter Yuumi then you need to take into account the best picks vs Yuumi and the worst picks vs Yuumi. This Yuumi counter data is based on matches played by Pros on patch 14.21 and previous ones. It is important to see whether Pros counter picked Yuumi or whether they played as Yuumi.
Champions used by Pros to counter pick Yuumi
Looking at the matches from Pros (see table Pros counter picked Yuumi) we can see that they picked 
Poppy most often to counter pick Yuumi. The matchup Poppy vs Yuumi occured 8 times. Yuumi has 6 wins and 2 losses in this matchup. Learn how to counter pick Poppy to get another perspective on the data.
Lane outcome for Yuumi
When looking at Yuumi 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 Yuumi. The lane outcome of Yuumi consists of three outcomes: Win, Loss or Draw. Let's take a look of Yuumi 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 Yuumi.
Conclusion
Analyze the available data and learn how to beat Yuumi. To master Yuumi take also a look on Yuumi builds and runes for Yuumi. Good luck, Summoner!