Equation for this graph - is it parametric?

[caleytm]

I can't seem to figure out the equation of the attached graph. I have tried polar, parametric, and cartesian, and I'm almost positive its parametric.
Does anyone know what the equation might be?

 

[ehild]

Look at the plot: x should be periodic, and y has to be combined from a monotonously increasing and a periodic part. Try something like x=sin(t), y=t+sin(2t) as in the figure attached and change constants to fit to the original one. ehild