This can be visualized easily using a variety of charts. Let's use Tag Cloud.

The steps are as follows:

1. Create a new visualization
2. Click on New and select Tag Cloud
3. Select logstash-* under From a New Search, Select Index

4. Set the bucket type to Tags
5. Select the Terms aggregation
6. Select the useragent.name.keyword field
7. Set the Size to 10 and click the Play (Apply Changes) button:

Save the visualization as Most used user agent. Chrome, followed by Firefox, is the user agent the majority of traffic is originating from.