add a colorbar axis label to cone plots

Quite used to making plots with colorbars in matplotlib and having the colorbar and colorscale calibrated to the quantity being plotted and giving it a label of the quantity and units.

Recently discovered plotly for 3D cone plots which is a perfect representation of the 3D vector field I am plotting, and I have it all scaled to the absolute values of those vectors. Just one thing missing - a label with units for the colorbar to make it totally obvious that this is quantitative and the viewer can read of the quantities from the colors in a unit they understand. But cannot find any documentation anywhere or even a forum post explaining such an apparently simple thing.

issue 中没有指定文件或测试。首先定位 cone plot 的 colorbar 实现，并确定其他地方如何配置 colorbar 标签；当用户可以提供一个量和单位标签，且该标签出现在渲染后的 colorbar 上时，即表示完成。