plotly / plotly/plotly.js

Mismatched Tooltip Behavior in Heatmap Charts with Line Chart Overlays

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bug P3
Dominant language
JavaScript
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Description

Issue Description:
When using a heatmap chart in Plotly.js, the tooltip functionality displays the "z" heatmap value corresponding to the "x" and "y" coordinates of the cursor when dragging over the chart:

image

However, when a line chart is drawn over the heatmap, the tooltip displays the "z" heatmap value corresponding to the point on that line instead of the heatmap data:

image

Expected Behavior:
The tooltip should display the "z" value from the heatmap data when dragging over the heatmap chart, even if other types of charts (like line charts) are present on the same plot.

Investigation Findings:
Upon further investigation, it was discovered that the findHoverPoints function inside Plotly's _hover function needs to be called without the winX and winY arguments (or they should be ignored) for this functionality to work as expected.

Steps to Reproduce:

  1. Create a heatmap chart with associated data.
  2. Add a line chart overlay to the heatmap chart.
  3. Hover the cursor over the heatmap and observe the tooltip data.

Additional Inquiry:

  • Could you please confirm if the tooltip behavior change when a line chart is present on the heatmap was intentional?
  • If this change was deliberate, could you provide insights or documentation on the reasoning behind this decision?
  • Is there any API or configuration option to customize the tooltip behavior based on the chart layers present?

Additional Information:

  • Plotly.js version: plotly.js-cartesian-dist, version: 2.32.0
  • Browser: Chrome Version 127.0.6533.88 (Official Build) (64-bit)

This issue affects the ability to accurately view data in complex charts with multiple layers. Any insights or resolution steps would be greatly appreciated. Thank you for your attention to this matter.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the heatmap and line-overlay case described in the issue using Plotly.js 2.32.0, then inspect the _hover function and its findHoverPoints call. Done means dragging over the heatmap reports the heatmap z value at the cursor rather than the overlaid line's point, with behavior checked against the reported case.

Written by the indexing model from the issue text.

Assessment

Tech stack
javascript
Domain
data-visualization
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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