I don't know how is on Mac, but the plotting is done by OMPlotWindow which is started by the simulation runtime.
Maybe OPENMODELICAHOME is wrong or so, probably Martin can tell you more.

You could use plot2/plotParametric2 instead, which is based on Java and should work fine.

Cheers,
Adrian Pop/

$ unset OPENMODELICAHOME && cat a.mo && cat a.mos && /opt/local/bin/omc a.mos 

class A

  Real x;

equation

  der(x) = time;

end A;

loadFile("a.mo");

simulate(A);

plot(x);

true

record SimulationResult

    resultFile = "A_res.plt",

    simulationOptions = "startTime = 0.0, stopTime = 1.0, numberOfIntervals = 500, tolerance = 1e-06, method = 'dassl', fileNamePrefix = 'A', storeInTemp = false, noClean = false, options = '', outputFormat = 'plt'",

    messages = "",

    timeFrontend = 0.004679566,

    timeBackend = 0.000658616,

    timeSimCode = 0.001480352,

    timeTemplates = 0.001502038,

    timeCompile = 0.770774324,

    timeSimulation = 0.017465982,

    timeTotal = 0.796675502

end SimulationResult;

true

The following shows the evil little OMPlotWindow. What tool are you using? MDT?

The evil little window remains elusive...

Wombat:~ lewis$ unset OPENMODELICAHOME
Wombat:~ lewis$ cd Models/junk/
Wombat:junk lewis$ /opt/local/bin/omc a.mos
true
record SimulationResult
    resultFile = "A_res.plt",
    simulationOptions = "startTime = 0.0, stopTime = 1.0, numberOfIntervals = 500, tolerance = 1e-06, method = 'dassl', fileNamePrefix = 'A', storeInTemp = false, noClean = false, options = '', outputFormat = 'plt'",
    messages = "",
    timeFrontend = 0.00562785,
    timeBackend = 0.000820135,
    timeSimCode = 0.001637659,
    timeTemplates = 0.001910501,
    timeCompile = 0.967065621,
    timeSimulation = 0.032840318,
    timeTotal = 1.010072317
end SimulationResult;
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct NULL not valid
Abort trap



I am using OMShell.  The environment variables work for OMEdit and MDT (excepting for TMPDIR in MDT, subject of recent post)