6 files declare their use of ChartBase in Charts 8.4

Billboard.php in modules/charts_billboard/src/Plugin/chart/Library/Billboard.php

C3.php in modules/charts_c3/src/Plugin/chart/Library/C3.php

Chart.php in src/Element/Chart.php

Chartjs.php in modules/charts_chartjs/src/Plugin/chart/Library/Chartjs.php

Google.php in modules/charts_google/src/Plugin/chart/Library/Google.php

Highcharts.php in modules/charts_highcharts/src/Plugin/chart/Library/Highcharts.php