The following plot shows an X-Y plot of Complex data (Inphase/Quadrature or Cosine/Sine) that was generated by ScopeDSP's Sine Generator. The Sine Generator produces a complex sine having a specified number of cycles in a specified number of points. In this case it generated three cycles in 256 points with a sample rate of 1 kHz. The plot is drawn using the "Dot And Line" format which puts a dot on each data point and connects them with a straight line.

Next, the same data is graphed in polar format
Naturally, a polar plot of a complex sine is a perfect circle:

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