How to show the full legend label as a tooltip on hovering over the legend symbol in vega lite

I am working on a use case where I want to display the full legend label as a tooltip when hovering over the legend symbol in Vega-Lite. The reason is that the legend labels are being truncated, so I want to show the full label name on hover as a workaround.

Here’s what I’ve tried so far:

I attempted to add a title attribute to the target HTML element using the following code:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Vega Legend Tooltip Example</title>
  <script src="https://cdn.jsdelivr.net/npm/vega@5"></script>
  <script src="https://cdn.jsdelivr.net/npm/vega-lite@5"></script>
  <script src="https://cdn.jsdelivr.net/npm/vega-embed@6"></script>
</head>
<body>
  <div id="vis"></div>
  <script>
    const vegaLiteSpec = {
      "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
      "data": {
        "values": [
          {"category": "Long Category Name 1", "value": 10},
          {"category": "Long Category Name 2", "value": 15},
          {"category": "Long Category Name 3", "value": 20}
        ]
      },
      "mark": "bar",
      "encoding": {
        "x": {"field": "category", "type": "nominal"},
        "y": {"field": "value", "type": "quantitative"},
        "color": {
          "field": "category",
          "type": "nominal",
          "legend": {"title": "Categories"}
        }
      }
    };

    vegaEmbed('#vis', vegaLiteSpec).then(result => {
      const view = result.view;

      // Wait for the Vega view to fully render
      view.runAsync().then(() => {
        // Select all legend symbol groups and add `title` attributes for tooltips
        const legendSymbols = document.querySelectorAll('.role-legend-symbol');
        
        // Array of category names for the legend symbols
        const categories = ["Long Category Name 1", "Long Category Name 2", "Long Category Name 3"];
        
        legendSymbols.forEach((symbol, index) => {
          // Set the full category name as the title tooltip
          symbol.setAttribute('title', categories[index]);
        });
      });
    });
  </script>
</body>
</html>

The code successfully adds the title attribute to the desired HTML elements, but I’m still not seeing the tooltip when hovering over the legend symbols.

Could anyone suggest a way to achieve this in Vega-Lite or Vega? I’ve read about using signals in Vega for user interactions but am not sure how to apply them here. Vega-Lite is preferred, but since I’m using JavaScript to render the chart, I can also manipulate the underlying Vega if necessary.

Thanks in advance!

Solution

Like this?

{
      "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
      "data": {
        "values": [
          {"category": "Long Category Name 1", "value": 10},
          {"category": "Long Category Name 2", "value": 15},
          {"category": "Long Category Name 3", "value": 20}
        ]
      },
      "mark": "bar",
      "encoding": {
        "x": {"field": "category", "type": "nominal"},
        "y": {"field": "value", "type": "quantitative"},
        "color": {
          "field": "category",
          "type": "nominal",
          "legend": {"title": "Categories"}
        },
        "tooltip": {"field": "category", "type": "ordinal"}
      }
    }

UPDATE

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "background": "white",
  "padding": 5,
  "height": 200,
  "style": "cell",
  "data": [
    {
      "name": "source_0",
      "values": [
        {"category": "Long Category Name 1", "value": 10},
        {"category": "Long Category Name 2", "value": 15},
        {"category": "Long Category Name 3", "value": 20}
      ]
    },
    {
      "name": "data_0",
      "source": "source_0",
      "transform": [
        {
          "type": "stack",
          "groupby": ["category"],
          "field": "value",
          "sort": {"field": [], "order": []},
          "as": ["value_start", "value_end"],
          "offset": "zero"
        },
        {
          "type": "filter",
          "expr": "isValid(datum[\"value\"]) && isFinite(+datum[\"value\"])"
        }
      ]
    }
  ],
  "signals": [
    {"name": "x_step", "value": 20},
    {
      "name": "width",
      "update": "bandspace(domain('x').length, 0.1, 0.05) * x_step"
    }
  ],
  "marks": [
    {
      "name": "marks",
      "type": "rect",
      "style": ["bar"],
      "from": {"data": "data_0"},
      "encode": {
        "update": {
          "fill": {"scale": "color", "field": "category"},
          "x": {"scale": "x", "field": "category"},
          "width": {"signal": "max(0.25, bandwidth('x'))"},
          "y": {"scale": "y", "field": "value_end"},
          "y2": {"scale": "y", "field": "value_start"}
        }
      }
    }
  ],
  "scales": [
    {
      "name": "x",
      "type": "band",
      "domain": {"data": "data_0", "field": "category", "sort": true},
      "range": {"step": {"signal": "x_step"}},
      "paddingInner": 0.1,
      "paddingOuter": 0.05
    },
    {
      "name": "y",
      "type": "linear",
      "domain": {"data": "data_0", "fields": ["value_start", "value_end"]},
      "range": [{"signal": "height"}, 0],
      "nice": true,
      "zero": true
    },
    {
      "name": "color",
      "type": "ordinal",
      "domain": {"data": "data_0", "field": "category", "sort": true},
      "range": "category"
    }
  ],
  "axes": [
    {
      "scale": "x",
      "orient": "bottom",
      "grid": false,
      "title": "category",
      "labelAlign": "right",
      "labelAngle": 270,
      "labelBaseline": "middle",
      "zindex": 0
    },
    {
      "scale": "y",
      "orient": "left",
      "grid": false,
      "title": "value",
      "labelOverlap": true,
      "tickCount": {"signal": "ceil(height/40)"},
      "zindex": 0
    }
  ],
  "legends": [{"title": "Categories", "fill": "color", "symbolType": "square",
  
  
   "encode": {

      "labels": {
        "interactive": true,

        "hover": {
       "tooltip":{"signal": "datum.value"}
        }
      }
    }
  
  }]
}