/**
 * Alter an individual chart before it is printed.
 *
 * @param $chart
 *   The chart renderable. Passed in by reference.
 * @param $chart_id
 *   The chart identifier, pulled from the $chart['#chart_id'] property (if
 *   any). Not all charts have a chart identifier.
 */
function hook_chart_alter(&$chart, $chart_id) {
  if ($chart_id === 'view_name__display_name') {
    // Individual properties may be modified.
    $chart['#title_font_size'] = 20;
  }
}