You can view this in the Data Explorer. Just select the custom event you want to look at, then click View Parameters. By setting the view type to ‘count’ you should be able to then see how many times that particular event occurred for that day on the graph.
But in my case I don’t have a custom event per level… In your example, you showed an event named level_2.
In my case, I have a single event “Level Started” and the Level Number is a Parameter of this event. Doing like you suggested, it shows me how much “Level Started” I had per day, but I need an additional option where it will group by the parameter “Level Number”, is that possible?
Currently there is no way to accomplish this with the way your custom events are set up. The easiest way to view the data would be to create a unique custom event for each level and then use the Data Explorer to view the day to day statistics. However, this would be a great way to improve the functionality of Unity Analytics. If you are interested you can vote for it to be added as a feature here, http://feedback.unity3d.com/forums/unity/suggestions?utf8=✓&status=0&category=analytics&view=hottest