Add value labels to charts

[hughess]

Chart Value Labels

This PR adds value labels to bar, line, and area charts.

Labels are turned off by default, but can be added by setting labels=true

This also includes a variety of props to format labels:

• labelFmt: works in the same way as our other fmt props and will override any other formats set
• labelColor: can pass a colour string in RGB, hexadecimal, CSS colour name, or "inherit" to take the colour of the series
• labelPosition:
  • Bar
    • outside
    • inside
  • Line / Area
    • above
    • below
    • middle
• labelSize: font size for labels

By default, label overlap is avoided, but there is an option to show everything:

• showAllLabels: boolean

In stacked bar charts, we’ve added a stack total label which appears above the bars. This can be turned off with:

• stackTotalLabel: boolean

Defaults

• labelColor: according to the colour of the series (ECharts manages colour contrast)
• labelFmt: same format as used for y column
• labelSize: 11pt, 1 smaller than axis labels
• labelPosition:
  • Single series bar: outside
  • Stacked bar: inside, with stack total label outside
  • Grouped bar: outside
  • Line: above
  • Area: above
  • showAllLabels: false
  • stackTotalLabel: true

Limitations

On stacked bar charts, the total label does not recalculate when hiding series by clicking on the legend. For now, I have turned off that type of interaction when a stacked total is present - otherwise, you could deselect a series and have the wrong total sitting on top of the bars.

This PR doesn’t include value labels for scatter plots, bubble charts, or histograms.

Scatter/bubble labels are up next, but involve some challenges because the approach will include the ability to pass a specific column to the label.

One issue that is unsolved here is the legend hover state. When you hover over a series in a legend, all labels will appear on the hovered series, including any overlap or conflicts. I've opened an issue with ECharts about this: https://github.com/apache/echarts/issues/19192

Questions

• Is “label” the right term here, or should these be called “valueLabel”?
  • Value labels is clearer, but makes all the props longer

Notes on changes to charting library

• This PR upgrades ECharts to 5.4.3 - this was needed to get a bugfix for line chart labels
• The legend has been changed to accept columns specified in each child chart component (e.g., Line.svelte) - this is because the hidden stack total series could not be removed from the legend otherwise
• Fixed #1108

Checklist

• [x] For UI or styling changes, I have added a screenshot or gif showing before & after
• [x] I have added a changeset

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[hughess]

Text Flickering Issue

Noticed this on Safari and checked out all the browsers I have. This reminds me of a previous issue I had with outlined text, where having a thin outline around characters caused some browsers to not render it correctly:

Mac Desktop

• Chrome: Works
• Firefox: Works
• Safari: Issue

Windows Desktop

• Chrome: Works
• Firefox: Works
• Edge: Works

Mobile

• Chrome: Issue
• Firefox: Issue
• Safari: Issue

Printing/PDF seems okay on all browsers as far as I can tell, it's just the hover interaction that seems to cause this

https://github.com/apache/echarts/issues/19199

[hughess]

Update on flickering issue

This is caused by the textBorder option in ECharts. We could turn it off, but it's very helpful in managing the contrast in labels compared to their backgrounds.

This issue only happens with the SVG renderer in ECharts. Switching to Canvas renderer solves the problem on desktop browsers. Still need to confirm for mobile browsers.

Should we change to the canvas renderer?

@ItsMeBrianD @csjh @mcrascal @archiewood

Initially I picked the SVG renderer because it maintains 100% crisp visuals when you zoom in vs. blurry with Canvas.

Here's ECharts' description of choosing a renderer:

Generally, Canvas is more suitable for charts with a large number of elements (heat map, large-scale line or scatter plot in geo or parallel coordinates, etc.), and with visual effect. However, SVG has an important advantage: It has less memory usage (which is important for mobile devices) and won't be blurry when zooming in.

I'm not sure how to check the performance/memory impacts of making the change.

Does anyone have concerns with changing to the canvas renderer?

[csjh]

I think some features are locked to certain renderers, though I'm looking through the docs now and the only one they mention is renderToSVGString (which we don't use)

[hughess]

UI test fails after changing to canvas because the test looks for a <g> element. Have tested the functionality and it's working

[hughess]

Realized this is going to need to work with the secondary y-axis as well.

That may mean changing the prop names to align to an axis. i.e.:

• yLabels
• yLabelFmt
• yLabelPosition
• yLabelColor
• yLabelSize
• yShowAllLabels
• yStackTotalLabel
• y2Labels
• y2LabelFmt
• y2LabelPosition
• y2LabelColor
• y2LabelSize
• y2ShowAllLabels
• y2StackTotalLabel

This may be confusing when compared to yAxisLabels and y2AxisLabels, so maybe adding "value" to these props is a good idea despite the longer names:

• yValueLabels
• yValueLabelFmt
• yValueLabelPosition
• yValueLabelColor
• yValueLabelSize
• yShowAllValueLabels
• yStackTotalLabel
• y2ValueLabels
• y2ValueLabelFmt
• y2ValueLabelPosition
• y2ValueLabelColor
• y2ValueLabelSize
• y2ShowAllValueLabels
• y2StackTotalLabel

[archiewood]

do you need to be able to control the two axes labels independently?

If you had two series on the same axis, you wouldn't be able to to that

Or could they both be controlled by one set of props?

labels
labelFmt
labelPosition
labelColor
labelSize
showAllLabels
stackTotalLabel

The only one I can see a reason to have independent control over is the fmt

[hughess]

I think fmt would be the most common, but I could also see position - e.g., you have a bar and want the labels to be positioned "inside" vs. "above" for the line on the other axis

[archiewood]

postion

you can't control position in a multi series line chart independently, so I would be happy to not support that.

fmt

with two axes there is a decent chance the formats would be different.

I'd probably have:

labelFmt - adjusts both yLabelFmt - adjusts first axis, overrideing labelFmt if specified y2LabelFmt - adjusts second axis, overrideing labelFmt if specified

[hughess]

good point - i had already forgotten that y2 only works with lines now. Agree with those