This dialog is available from the Edit / Trendline... menu command under the Graphs window. It is also available at Edit / Investment Data / Trendline... in the Portfolio Editor window, or by right clicking on any investment and choosing the Investment Data / Trendline... popup menu command.
Use this dialog to numerically enter the starting and ending positions of the trendline. Also select the graph type that the trendline will be drawn on. If it is a graph type that has an upper/lower section you will also have the option to choose which section to display the trendline on. When specifying the starting/ending position the Yield figure is updated to reflect your latest entry.