[OS X TeX] parametricplot in pstricks

[Luis Sequeira]

The problem is one of scale. Since cos expects degrees, and your t goes from -1.8 to 1.8, 
your   this is practically a straight line. 
I mean cos(0.25πt) when t is 1.8 degrees is approx 0.99969... 

Bottom line, the output is correct.

You may try changing the interval, and also, for good measure, use different scales for x and y.

Try this for comparison:

\parametricplot[plotstyle=curve, plotpoints=50,yunit=0.05mm,xunit=1cm]{-360}{360}{0.25 t Pi mul mul cos t}

Luis Sequeira