Make the legend extra parameter customizable

[markov00]

Is your feature request related to a problem? Please describe.
The current legend extra parameter is used to display the latest value on the data series, or the hovered data point value. We should make this configurable to show other values or statistics, like the total value, averages etc

Describe the solution you'd like
We should move the legend props to it's own <Legend> component and allow the user to customize in some way the possible extra parameter

Describe alternatives you've considered
n/a

Additional context
continuation of #246

Kibana Cross Issues

• [Lens] show max and min in charts' legend like Splunk does kibana#19485
• Metrics values in chart’s legend kibana#16096
• legend tooltip should display metrics kibana#10524
• Visualizations and dashboards need basic result count information kibana#6279

Checklist

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present

[formgeist]

@markov00 @nickofthyme Not sure about your prioritization for 7.11, but this is another much-needed addition to the charts in order to display averages across selected time ranges in APM. Can you share an update on this? Thanks 👍

[ghudgins]

+1 o11y case

I guess I expected it to show an aggregate value of the data, average of the entire data for average, max for the maximum line item, etc.

[PhilippeOberti]

+1 for the @elastic/protections-experience team, we could use a feature to show accumulated values when the user isn't mousing over anything in the chart.
I see that the Alerts page currently has this feature, but the developers had to build an entirely new Legend component themselves.

[yannbolliger]

It would be great if one could provide a custom legend component like for the tooltip.

[yannbolliger]

It would be great if one could provide a custom legend component like for the tooltip.

@markov00 would you accept a PR if I was to try and implement this?

[markov00]

Hi @yannbolliger such a task brings in a set of decisions that we should make before attempting an implementation:

• What are we looking to expose in the API to provide this capability?
• Is the current Legend Item object enough or does it need more details (the data table view related to such item for example)?
• Do we want to expose full control over the value (giving the freedom to the user to send a custom function) or a set of predefined operations (sum, avg, last, min, max)?
• How can we improve the legend style to accommodate correctly such metric? Today we don't describe such value, but a header will help a lot the readability of the legend.
• Who should provide the formatting and how do we want to format such value? Does the legend needs to shrink/grow depending on that?
• Is a value column enough or do we want to transform the legend to a table to allow multiple stats?
• Does that value change with interactions (as it is today) in all cases? if so what will change to inform the user?

The current legend extra parameter was not actually well thought out when it was merged. It was pushed into the library to align the feature with a legacy functionality that we have in Kibana, so that is why I'm a bit hesitant about letting you try an implementation (I just don't want to waste your time and not accept the PR due to its implementation/logic).
I'm very happy btw if you are interested in discussing this feature and how we can structure a good logic behind that.

[nickofthyme]

There are also implications related to the width of the legend, such that allowing a fully custom element would require rethinking how we currently size the legend and legend items. Currently, we use CanvasRenderingContext2D.measureText() here to determine the max width of all labels + extra value strings to appropriately size the legend all while limiting reflows.

[yannbolliger]

I'm very happy btw if you are interested in discussing this feature and how we can structure a good logic behind that.

@markov00 @nickofthyme Thanks for the replies! I'm also interested in discussing this.
Is there some easier way to discuss this than Github issue comments?

[markov00]

@nickofthyme I've talked this morning with Yann and we moved the request toward #862

[markov00]

Reopening since the need of making the "extra" parameter configurable is still there

[markov00]

Hey @formgeist, our dear @gvnmagni has explored a bit this space and how it will probably look like. Please take a look at #580

[formgeist]

Hey @formgeist, our dear @gvnmagni has explored a bit this space and how it will probably look like. Please take a look at #580

@markov00 sorry if I'm missing something, but is there a specific issue in that list of features that you're referring to?

[markov00]

@formgeist sorry wrong link for you: #2096