Creating More Complex Reference Lines
In Looker, you can create reference lines as described in this document. However, you may instead be looking for something more specific in the reference line, like weighted average. If this is the case, the best way to create a weighted-average reference line would be to create a table calculation that calculates a weighted average the way you want.
For example, if you wanted to weight average user age, by the count of users, you could add both user count and average age to an Explore. Then, you could create a table calculation like this:
This table calculation will return a single value—the weighted average—for every row of the query.
Then, you can make this table calculation a chart, as a line using series types. Since the table calculations only returns one value, the line will be a horizontal reference line. If you do not want users count to appear in your chart, you can also hide that field using the Hide from Visualization option.
Alternatively, you can create a weighted-average measure in LookML as shown in this document.