However, I'm unsure whether some of the changes are necessary or appropriate.

Hi @CoderPM2011, I took a quick look and this is looking great! Feel free to create whatever types and interfaces feel right, and I can refine them as needed right before I merge. Let me know when you think this is ready to merge and I'll make those edits and add it to the cookbook! 🚀

Note that the test are failing because the new classes in the Primitives folder should just have the namespace ScottPlot and not ScottPlot.Primitives. Similparly, the new interfaces in the Interfaces folder may be in the ScottPlot namespace, not ScottPlot.Interfaces. Fixing this will probably make the tests pass 👍

I can refine them as needed right before I merge

I'll make those edits and add it to the cookbook

If you'd rather, I can make some refinements myself by editing the code in this PR, then let you know when I'm done so you can pick it up again. I don't want to start making commits here without you being ready for it though, because merge conflicts inside pull requests can get pretty annoying 🤪

Take as long as you need! I'm putting a lot of time today into #3186

The test code is at the bottom.
It is currently possible to adjust the style of any axis line.

I'm not sure if the current axis naming is suitable or easily understandable.

My test code:

var pts = new PolarCoordinates[] {
    new PolarCoordinates(10, 45),
    new PolarCoordinates(50, 90),
    new PolarCoordinates(30, 127),
    new PolarCoordinates(15, 270),
    new PolarCoordinates(125, 180),
};

ScottPlot.Plottables.Polar polar = formsPlot1.Plot.Add.Polar(pts);
ScottPlot.Color[] map = new ScottPlot.Colormaps.Turbo()
    .GetColors(polar.CircularAxis.AxisLines.Length);
polar.RadialAxis.Spokes[1].LineStyle.Pattern = LinePattern.Dotted;
polar.RadialAxis.Spokes[1].LineStyle.Width = 3;
polar.RadialAxis.Spokes[1].LineStyle.Color = Colors.Red;

polar.RadialAxis.Spokes[2].LineStyle.Pattern = LinePattern.Dashed;
polar.RadialAxis.Spokes[2].LineStyle.Width = 2;
polar.RadialAxis.Spokes[2].LineStyle.Color = Colors.Silver;

polar.RadialAxis.Spokes[3].LineStyle.Pattern = LinePattern.DenselyDashed;
polar.RadialAxis.Spokes[3].LineStyle.Width = 5;
polar.RadialAxis.Spokes[3].LineStyle.Color = Colors.Purple;

for (int i = 0; i < polar.CircularAxis.AxisLines.Length - 1; i++)
{
    polar.CircularAxis.AxisLines[i].LineStyle.Pattern =
        (i % 2 == 0)
        ? LinePattern.Dotted
        : LinePattern.DenselyDashed;
    polar.CircularAxis.AxisLines[i].LineStyle.Width =
        (i % 2 == 0)
        ? 3
        : 1;
    polar.CircularAxis.AxisLines[i].LineStyle.Color =
        map[i].WithLightness(0.3F);
}
polar.LegendText = "TEST";

var RadialAxisPt = Coordinates.FromPolarCoordinates(130, 300);
for (int i = 0; i < 4; i++)
{
    var RadialAxisPt2 = Coordinates.FromPolarCoordinates(70, 90 * i);
    formsPlot1.Plot.Add.Callout(
        "RadialAxis",
        textLocation: RadialAxisPt.WithDelta(+50, 0),
        tipLocation: RadialAxisPt2);
}

var CircularAxisPt = Coordinates.FromPolarCoordinates(125, 140);
for (int i = 1; i <= 4; i++)
{
    var CircularAxisPt2 = Coordinates.FromPolarCoordinates(125.0 * i / 4.0, 140);
    formsPlot1.Plot.Add.Callout(
        "CircularAxis",
        textLocation: CircularAxisPt.WithDelta(-150, 0),
        tipLocation: CircularAxisPt2);
}

formsPlot1.Update();

Hi @CoderPM2011, thanks so much for this PR! It took me a while to get to because I was focused entirely on #4053 for the last few weeks, but now that it's merged I'm happy to pick this up now. I'll probably make a few refinements, then merge it just like it is. This is looking great! 🚀

Heads up, the two major change I'm about to start pushing to this branch are:

1. Adding cookbook recipes that document much of the functionality. I'm refactoring lightly as I go to maximize simplicity of these recipes.

2. I may refactor the the "polar plot" (which stores coordinates) to act more like a "polar axis" which only stores information about spoke and circle axis lines. It would have methods to return cartesian coordinates given polar coordinates, then users can use it with all the existing plot types (not just markers, but also lines and polygons)

Still a work in progress, but you can probably see the direction this is going by looking at the new cookbook recipes!

Thanks again for your work on this! It's looking fantastic! 🚀

Following-up, I removed some of the functionality from a few of the types that wasn't used (equality checks, implicit operations between polar coordinates and cartesian coordinates, etc.) but if you think it would be useful to other peoples' workflows (code outside ScottPlot) let me know and we can add some of that back in.

Thanks again for this PR @CoderPM2011! It was a pleasure to review today 😄

I'm going to merge it now, but feel free to open new PRs if you recommend refinements and I'll merge those in quickly 🚀