[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!

[robertclaus]

@LloydLS thank you for the suggestion!

We found this example online of similar behavior in another tool. Is this what you would be looking for?

https://demonstrations.wolfram.com/BasicTernaryPhaseDiagram

[LloydLS]

Hi @robertclaus

Many thanks for considering my suggestion!

Yep, your image demonstrates nicely the spikelines mapping to the axes in an anticlockwise direction (for when the BC axis increases from left to right). Clockwise is also valid but essentially "rotates" the spikelines by -60 degrees (for when the BC axis decreases from left to right).

I'm basically looking to replicate the (really nice) spikeline functionality that works on x and y axes, but in a ternary plot as well.

(Hope I've explained that well enough, please let me know if not)

Best regards

[robertclaus]

@LloydLS that makes sense! I don't know when we would get to this in terms of priority given our small team - but we agree this would be a great feature.

One note for anyone that wants to give this a shot - this config would probably be better in layout.ternary.annotations since it's likely to have slightly different options than traditional cartesian plots. Especially if we want to extend the idea to other non-cartesian coordinates like polar, carpet, 3D, etc. in the future.

[LloydLS]

I wonder whether actually it would be better to put the spikeline properties under layout.ternary[aaxis|baxis|caxis] instead of layout.ternary.annotations as the former is analogous to the spikeline properties available under layout[xaxis|yaxis] for scatter plots?

[Lexachoc]

Hi, I've already implemented this (though only as a prototype).

Is this what you're looking for?

ref: https://community.plotly.com/t/hovermode-spikemode-for-ternary-diagram-ternary-plot/82352/2

[LloydLS]

Hi @Lexachoc ,

Those look nice as clickable annotations on the plot.
I'm more after the hover lines to the axes; direct analogues to the showspikes (etc) properties that are available under layout.xaxis and layout.yaxis in Cartesian plots.

Here's a CodePen I found that shows the plotly spikelines functionality:

https://codepen.io/p1ho/pen/VdJOZB

Best regards,

[Lexachoc]

Hi @Lexachoc ,

Those look nice as clickable annotations on the plot. I'm more after the hover lines to the axes; direct analogues to the showspikes (etc) properties that are available under layout.xaxis and layout.yaxis in Cartesian plots.

Here's a CodePen I found that shows the plotly spikelines functionality:

https://codepen.io/p1ho/pen/VdJOZB

Best regards,

Hi @LloydLS ,

Sorry, I forgot to mention that my implementation is exactly for the hover effect.

PixPin_2026-08-24_14-51-22.mp4

I think I could try to submit a PR (draft) so others can take a look and suggest improvements. I'm a little worried about performance, though. (I haven't tested it yet.)

But the PR might not come so soon, since I'm pretty busy this month.

[LloydLS]

Oh, very nice! Yes, that's precisely what I'm looking for.

Many thanks!

[robertclaus]

@Lexachoc that looks great! We would love to review the PR whenever you have a chance.