[FEATURE]: Spikelines in scatterternary plots

[LloydLS]

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!