Open a pipe to a gnuplot process for visualization of images and control values.
gnuplot_open_pipe opens a pipe to a gnuplot sub-process with which subsequently images can be visualized as 3D-plots (gnuplot_plot) or control values can be visualized as 2D-plots (gnuplot_plot_ctrl). The sub-process must be terminated after displaying the last plot by calling gnuplot_close.
There can only be one gnuplot output-stream open at any given time. If gnuplot_open_pipe when a gnuplot output stream is open, the old output stream is closed first.
gnuplot_open_pipe returns the value TRUE if the sub-process could be created. Otherwise, an exception is raised.
gnuplot_plot, gnuplot_plot_ctrl, gnuplot_close