ternary_contour

Hi,

The latest release notes (at least the past two, I believe) at plotly.py mention the introduction of ternary contour plots using type = 'ternary_contour', which i'm guessing is similar to geom_density_tern in ggplot. I'm not sure if it will eventually make it into the R implementation of plotly without asking, but it's something that would be great to have. Right now the 'scatterternary' type is the only option for ternary diagrams and it's only suitable for making simple plots. Is this on the list of things to add?

Discussion is here:
https://github.com/plotly/plotly.py/pull/1413

plotly.py の pull request 1413 の議論を確認し、plotly.R に既存する scatterternary のサポートと、要求されている ternary_contour 機能を比較してください。R 実装の想定される範囲を確認し、現在の三元図に加えて、動作する三元等高線プロットがサポートされることを完了条件として定義してください。