I do not believe this is available in the UI today. Let me see if there is another way to do this.
I think you could try to make use of custom resources, select "custom chart" and then configure that chart as per your requirements.
Select configure this resource and pick the data source which you want to report on the chart, in your case "Transaction".
Finally pick the metric you want on the chart Y-axis or X-axis by clicking on DATA SERIES.
This way you will have only "Avg. Duration" on the chart. Hope this procedure fulfills your requirement.