Using property <strong>gradientRatio</strong> you can achieve any gradient you want. This particular chart uses the following gradient ratio: <strong>[-0.4, -0.4, -0.4, -0.4, -0.4, -0.4, 0, 0.1, 0.2, 0.1, 0, -0.2, -0.5]</strong>
Negative value means the color will be darker than the original, and positive number means the color will be lighter. <strong>gradientRatio</strong> array can be of any length you want, depending on a gradient you want to achieve.
Pie chart also supports linear gradient. To enable it set <strong>gradientType</strong> to <strong>linear</strong>. However, radial gradients would look much better.