message: fix(legacy-plugin-chart-horizon): error on missing groupby
diff: import React from 'react';
  import PropTypes from 'prop-types';
  import { extent as d3Extent } from 'd3-array';
  +import { ensureIsArray } from '@superset-ui/core';
  import HorizonRow, { DEFAULT_COLORS } from './HorizonRow';
  import './HorizonChart.css';
  @@ -85,7 +86,7 @@ class HorizonChart extends React.PureComponent {
  key={row.key}
  width={width}
  height={seriesHeight}
  - title={row.key.join(', ')}
  + title={ensureIsArray(row.key).join(', ')}
  data={row.values}
  bands={bands}
  colors={colors}
  