function getLayerVisualChannels(layerType, colorByFeature) {
  // todo: derive default visual channels from layerType
  switch (layerType) {
    case 'heatmap':
      return {
        weightField: null,
        weightScale: 'linear',
      };
    case 'hexagonId':
      return {
        colorField: {
          name: colorByFeature.name,
        },
        colorScale: 'quantize',
      };
  }
}
@param {String} layerType kepler layer type
@param {Object} colorByFeature field definition by which to color visual layers, contains {name, type, tableFieldIndex}
@returns visualChannels field of kepler config