How to counter Diana
Let us take a look on how to beat Diana. We will look at champions who are strong against Diana and who are weak against Diana. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Diana. You can already win in champion select with the right counters for Diana. If you want to know how to counter Diana then you need to take into account the best picks vs Diana and the worst picks vs Diana. This Diana counter data is based on matches played by Pros on patch 14.12 and previous ones. It is important to see whether Pros counter picked Diana or whether they played as Diana.
Champions used by Pros to counter pick Diana
Looking at the matches from Pros (see table Pros counter picked Diana) we can see that they picked 
Tristana most often to counter pick Diana. The matchup Tristana vs Diana occured 25 times. Diana has 15 wins and 10 losses in this matchup. Learn how to counter pick Tristana to get another perspective on the data.
Lane outcome for Diana
When looking at Diana 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 Diana. The lane outcome of Diana consists of three outcomes: Win, Loss or Draw. Let's take a look of Diana 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 Diana.
Conclusion
Analyze the available data and learn how to beat Diana. To master Diana take also a look on Diana builds and runes for Diana. Good luck, Summoner!