Incomplete legend for multiple colored bars in one trace

[hy9be]

I noticed that Plotly.js construct legend item per trace rather than per color: https://github.com/plotly/plotly.js/blob/master/src/components/legend/get_legend_data.js#L43-L71. Thus if markers in a trace are colored differently, the legend could only show the color of the first item.

Here is an example:

I tried to make some changes to draw the legend items correctly. But the filtering functionality breaks. And I failed to find a way to fix the filter as the current implementation is based on the concept of "trace".

[etpinard]

You're correct. The current behavior isn't great and is far from perfect in every situation. But, can I ask, what's the desired behavior in your mind? I don't think we should try plotting a rainbow-like legend item with the colors of all the trace's points. Maybe we could hash the legend item? Something else?

[alexcjohnson]

from @r1z1a in #1634 - there the use case was a trace that started all one color but during interactions one or another point (bars in that case too, though this applies to at least scatter as well) would change. @r1z1a 's suggestion:

An Ideal would be to store legend color when you first time plot it and pass data in, and after that the restyle shouldn't take effect if you are re-styling just single bars.

It's important that whatever solution we end up with depend only on the current state, not on its history, so while this suggestion is tempting as it produces the "right" result in that specific case, it's not viable in general.

One way we could solve this would be to explicitly specify legend styling. In principle this would need to be fairly involved, replicating all the marker, marker.line, and (for scatter) line attributes, but of course as a user you would only need to specify those attributes for which we get the default wrong.

[r1z1a]

@alexcjohnson
Another suggestion would be to have some sort of Boolean when you are creating a trace to have something like "legendlock" - if set true this will prevent the legend from being altered.

From my perspective the legend color should stay the original color plotted and only if you re-style whole trace it should then be changed.

You say you want to depend on current state of the trace, so whats the current state of a trace with 100 scatter markers that are colored red and green for instance. Should a trace legend be an Array of colors used for the plot? Should it be the First color? Maybe the color that appears the most in the trace.

I personally think this is the whole restyle() issue. Its great when you want to change whole trace, but when it comes to single points in a trace, it can become very hard, and fair few issues are coming from that.

[marcinwojciechowski]

Another example (If I understand the problem here) is that I would like to compare 2 properties: temperature and pressure. Each property should be assigned to its own axis (because of the property unit - tempearature unit (Celsius) and pressure unit (Pascal)).
So the data is:
tempearature: source A values is 32, source B value is 35
pressure: source A value is 1014, source B value is 996

So how can I show barchart with legend groupped by sources (A and B)? Each source should have its own color and every property that comes from that source should have the color of its own source.

Like in this example: https://codepen.io/morris4life/pen/eepZNV?editors=0010
But the legend instead of showing every trace it should show just 2 items: "Source A" and "Source B"

[etpinard]

Now in #3735