How to use the vega-scale.scheme function in vega-scale

function configureScheme(type, _, count) {
  var extent = _.schemeExtent,
      name, scheme;

  if (isArray(_.scheme)) {
    scheme = interpolateColors(_.scheme, _.interpolate, _.interpolateGamma);
  } else {
    name = _.scheme.toLowerCase();
    scheme = getScheme(name);
    if (!scheme) error(`Unrecognized scheme name: ${_.scheme}`);
  }

  // determine size for potential discrete range
  count = (type === Threshold) ? count + 1
    : (type === BinOrdinal) ? count - 1
    : (type === Quantile || type === Quantize) ? (+_.schemeCount || DEFAULT_COUNT)
    : count;

  // adjust and/or quantize scheme as appropriate
  return isInterpolating(type) ? adjustScheme(scheme, extent, _.reverse)
    : isFunction(scheme) ? quantizeInterpolator(adjustScheme(scheme, extent), count)
    : type === Ordinal ? scheme : scheme.slice(0, count);
}