There is no way to set marker(s) from FPScript, but there are other ways to achieve the same goal.
Let your FPScript formula extract the desired points as a signal. Add it to the diagram. Using the Properties window of the new curve, switch off the Connection Line and activate the Data Labeling. In the Text field enter
Y = %
X = %
(you may add some formatting too, e.g. %.3f for three decimals).
Change the Angle to 0°, the Position to Top Right and the Background to the desired color. If you want to see the points too, turn on the Symbols and choose the appropriate Type and Colors.
