Select value for trendline

With the update of April it is possible to add a trendline. However, take this example: You have a line chart with 3 separate lines in it. When you turn on the trendline option, the chart will show a trendline based on the values of all three lines, while I only want the trendline to be based on one of those three lines. I think it would be a great idea to add the option to select the value/values the trendline should be based on. Thanks!