This example shows a donut chart with a custom circular axis that displays proportional values and fee percentages for different investment types. The visualization uses a radial scale implemented through the `d3-scale-radial.js` module, which wraps `d3.scaleLinear` to map numeric values to radial distances, while `d3.pie` and `d3.arc` generate the donut segments. The custom axis places tick marks and labels around a circle at 5% intervals, with labels flipping orientation for values above 0.5. The code also renders fee data as circles positioned using trigonometric calculations based on the linear x-scale, with styles filled from an ordinal color scale.