[FEATURE]: Spikelines in scatterternary plots

Hi,

Description

It would be great if we could add spikelines to scatterternary plots, mapping to the a,b,c axes in an analogous way to the spikelines in a scatter plot.

Why should this feature be added?

Adding this feature would help the students we support become proficient in reading data off this plot type.

{
  "ternary": {
    "aaxis": {
      "showspikes": true,
      "spikecolor": "",
      "spikedash": "",
      "spikemode": "",
      "spikesnap": "",
      "spikesthickness": ""
    },
    "baxis": {
      "showspikes": true,
      "spikecolor": "",
      "spikedash": "",
      "spikemode": "",
      "spikesnap": "",
      "spikesthickness": ""
    },
    "caxis": {
      "showspikes": true,
      "spikecolor": "",
      "spikedash": "",
      "spikemode": "",
      "spikesnap": "",
      "spikesthickness": ""
    }
  }
}

Many thanks!

まず、既存の scatter plot の spikeline の動作と、scatterternary plot における a、b、c 軸の処理を比較します。scatterternary plot が要求された spike 設定をサポートし、scatter plot と同様の spikeline を表示できれば完了です。