In your view file

<%= column_chart(async_traffic_forecast_path(resource, params[:id]),
                 library: {
                   legend: { position: 'top' },
                   scales: {
                     'y' => {
                       "type" => 'linear',
                       "display" => true,
                       "position" => 'left'
                     },
                     'another-y-axis' => {
                       "type" => 'linear',
                       "display" => true,
                       "position" => 'right'
                     }
                   }
                 }
    ) %>

in your controller

traffic_data_response = [
  { data: views, name: "Views", library: { yAxisID: "y" } },
  { data: other_views, name: "Other Views", library: { yAxisID: "y" } },
  { data: total_conversion, name: "Conversion", library: { yAxisID: "another-y-axis" } }
  { data: other_conversion, name: "Other Conversion", library: { yAxisID: "another-y-axis" } }
]