you'll need to be on splunk version 6.6 to use the trellis option. https://docs.splunk.com/Documentation/SplunkCloud/6.6.0/Viz/VisualizationTrellis
When you're on the Visualization tab, you'll see a Trellis button next to Format. When you click on it, you'll see a checkbox for Use trellis layout. There is a Split by option and you can choose to split by various available fields or the aggregation of your fields.
There is a syntaxing note that I'll forewarn you about, and it's that I recommend you end the syntax with a command like stats, chart, timechart, etc. and not an eval command, because the split by option sometimes will only allow you to split by the aggregation in those cases.
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