Drupal::moduleHandler()->loadInclude('flot_d8', 'inc', 'flot_d8.theme');
function flot_d8_theme(array $existing, $type, $theme, $path) {
  return [
      'flot_d8' => [
        'file' => 'flot_d8.theme.inc',
      ],
    ],
  ];
}

function template_preprocess_views_view_flot_d8(&$variables) {
  $options = $variables['view']->style_plugin->options;
  $series = [];

  foreach ($variables['view']->result as $id => $result) {
    $label = $result->labels;
    $numbers = $result->numbers;
    $series['data'][] = [
      'label' => $label,
      'data' => $numbers,
    ];
  }

  $options['series'] = $series;

  // Update options for twig.
  $variables['options'] = $options;
}

$output[] = [
  '#type' => 'flotd8',
  '#theme' => 'flot_examples_series_pie',
  '#data' => $variables['options']['series'],
  '#options' => ['series' => ['pie' => ['show' => TRUE]]],
];
return $output;

function theme_views_view_flot_d8($variables){
  $output[] = [
  '#type' => 'flotd8',
  '#theme' => 'flot_examples_series_pie',
  '#data' => $variables['options']['series'],
  '#options' => ['series' => ['pie' => ['show' => TRUE]]],
];
return theme('flotd8', $output); //This doesn't work