Automated coloring calculation - interval, percentile

Motivation

Calculating the right values for color steps is hard and error-prone. Why not using mapbox gl to take over this task?

Design

Current approach

  map.addLayer({
    'id': 'data_layer',
    'source': 'data',
    'type': 'fill',
    'paint': {
        'fill-color': {
          property: 'my_field',
            type: 'interval',
            stops: [
              [0.002, '#dedcf1'],
              [0.015, '#bfbce5'],
              [0.0276, '#9c98d7'],
              [0.0394, '#6760c1'],
              [0.05, '#1f12a0']],
            default: "#ffffff"
        }
    }
  });

New approach

  map.addLayer({
    'id': 'data_layer',
    'source': 'data',
    'type': 'fill',
    'paint': {
        'fill-color': {
          property: 'my_field',
            type: 'interval', //'interval' or 'percentile'
            colors: ['#dedcf1', '#bfbce5', '#9c98d7', '#6760c1', '#1f12a0'],
            default: "#ffffff"
        }
    }
  });

Instead of pairing each color with a value, in this approach Mapbox will calculate it by itself. By setting the type to interval, Mapbox will look at all values, the numbers of colors and calculating the steps by itself.

Types

Not only interval should be a possible option for type, I would recommend the following:

• Interval
• Percentile

Pros and Cons

I cant talk about the complexity to implement it. One contra: Performance. Imagine we will use automated coloring for fill-color and line-color. The colors should in this case only be calculated once. Therefore it should possible to store the result of the calculation in a variable.

No files or tests are named. Start by tracing the map.addLayer handling for fill-color expressions, then determine how interval and percentile values would be calculated from the available feature data. Done means supporting the proposed colors and type options while allowing one calculation to be reused by fill-color and line-color.