How can i fix it.
The problem is that for interactive plot to work, you need octave running and the readline loop running, as part of the communication between the GUI and octave occurs in the readline event loop. In the case of FLTK backend, the whole GUI event loop occurs in the readline event loop. When you start octave in embedded mode and just run eval_string("plot(...)"), octave exits immediately and kills any plot.
So you need to pause octave interpreter after the plot command. What might work is to call the octave "pause" command.
Michael.