The alternative would be a third-party library like PLplot, which does provide a Fortran interface. You'll need to build that as well on macOS first, though, or use a package manager like Homebrew or MacPorts to install it as a binary.
There is a library called fplot that allows for interacting with GNUPlot from within Fortran. I've never personally used it, but it looks promising.
]]>How do I do that in Simply Fortran for the Mac? I only need 5 routines: Open a plotting window, change the pen color, move the pen, draw a line, and close the window.
Simply Fortran ships with a plotting package, but it does not support running graphs or closing/replacing existing ones on macOS, so that won't work. You would need a third-party plotting library.
I do not think that will work for me. Even if the data file is linked to gnuplot, the data does not appear in the data file when fortran executes a write statement, it is held in a buffer, sometimes until the program terminates.
Indeed the output is buffered. You can, however, use the GNU extension FLUSH to force a unit to flush to disk or screen (depending on the unit). Writing to a data file that GNUPlot reads might be preferable to trying to directly integrate or call a library just due to the simplicity.
]]>