theta and theta2 chanel encoding by field

Question

I want to create arcs using vega-lite, here you will find a working example: enter link description here

but this does not work: enter link description here

The difference is only that I use this in the working example

"encoding": {
"theta": {"value": {"expr": "datum.thta"}},
"theta2": {"value": {"expr": "datum.thta2"}}  }

And this is the code that does not work:

"encoding": {
"theta": {"field": "thta"}, 
"theta2": {"field": "thta2"}}  }

Can someone explain why using the "fields" creates a half circle (not wanted) and "value" creates the quarter circle (wanted)?

Many thanks in advance

user_2937831 · Answer

When you specify values via encodings, the scale is automatically adjusted based on the content of the data. You can fix this by specifying that the scale should be from 0 to 2pi (open in editor):

{
  "width": 80,
  "height": 80,
  "params": [{"name": "radius", "value": 0}, {"name": "radius2", "value": 50}],
  "data": {
    "values": [{"name": "arc1", "quadrant": "TopRight", "ring": "Hold"}]
  },
  "transform": [
    {
      "calculate": "if(datum.quadrant === 'TopRight'  , PI*0.5 , null)",
      "as": "thta2"
    },
    {"calculate": "if(datum.quadrant === 'TopRight'  , 0 , null)", "as": "thta"}
  ],
  "mark": {
    "type": "arc",
    "radius": {"expr": "radius"},
    "radius2": {"expr": "radius2"}
  },
  "encoding": {
    "theta": {"field": "thta", "scale": {"domain": [0, "2*PI"]}},
    "theta2": {"field": "thta2"}
  }
}

theta and theta2 chanel encoding by field

1 Answers