How to encode a field using conditions in Vega-Lite

[nickjwheatley]

OBJECTIVE

Hi Folks, I'm trying to add data labels to my chart, however I have multiple bars layered on top of each other, and I need the data labels to hover over different bars depending on if values in a certain field are positive or negative.

ATTEMPT

This could be achieved by changing the "field" property of the "y" encoding using a condition. I've spent some time exploring Vega-Lite documentation and experimenting with some stuff, but I can't get it to work no matter what I try. Vega seems to ignore by condition. I'm also curious if I'm able to apply conditions to "mark" rather than "encoding". When values are negative, I'd like to change "dy" to 10 from -10.

Any suggestions?

 "mark": {
    "type":"text",
    "dy":-10
  },
  "encoding": {
    "text": {
      "field": "field_one"
    },
    "y": {
      "condition":{
        "test":"datum['test_data'] < 0",
        "field": "field_one"
      }, 
      "field": "field_two"
    }....
  }

[chanwutk]

For your dy question, would you mind checking this example?

For the other question, would you mind trying to transform with calculate before encoding?

Here is an example

  "mark": {
    "type":"text",
    "dy": {"expr": "datum.longer_bar_value < 0 ? 10 : -10"}
  },
  "transform": {
    {
      "calculate": "Math.abs(datum.thin_bar_value) > Math.abs(datum.thick_bar_value) ? datum.thin_bar_value : datum.thick_bar_value",
      "as": "longer_bar_value"
    }
  }
  "encoding": {
    "text": {
      "field": "longer_bar_value"
    },
    "y": {
      "field": "longer_bar_value",
      "type": "quantitative"
    }....
  }

Please let us know if you have any question :)