There are a few options here, depending on whether the target data lives in your database, and how granularly the target data needs to be applied.
The simplest target setting mechanism is to use a reference line in a graph for a single target.
You can also encode targets in LookML and use the HTML parameter to custom format or add images, such as an arrow, depending on an IF statement.
If your target data lives in the database and applies to specific rows or aggregations from your performance data, you can also reference that target data in your IF statement above. See “Accessing values from other fields” in the documentation for the HTML parameter.