I have no experience with this plugin, but after a look at the source code, I figured it out.
You have to call the plugin for each curve with a single equation like this:
Then use the cos() equation and the sin() + cos() equation with same start/end/step parameters.
With two equations the first one is used for X values and the second for Y values.
This explains your result, you have a circle with radius 100.
Check this with start=0, end=180, this results in a half circle.
Armin
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