I am coding a new visualforce chart and need help with the Controller... I am new to Apex so I appologize if the fix is something basic but I am unable to figure this out. Here is my code:
public class GaugeChartController {
    public String acctId {get;set;}

    public GaugeChartController(ApexPages.StandardController controller){
        acctId = controller.getRecord().Id;
    }

    public List<gaugeData> getData() {
                  
   AggregateResult SurveyResponses = [select Account__c, Count(Name), avg(Liklelihood_of_recommending_AECOM__c) npsscore
                                   from NPS_Survey_Response__c
                                   where Account__c =: acctId
                                   group by Account__c];

          List<gaugeData> data = new List<gaugeData>();
          data.add(new gaugeData(Integer.valueOf(SurveyResponses.get('npsscore'))));
           return data;
      }
    public class gaugeData {
        public String name { get; set; }
        public Integer size { get; set; }

        public gaugeData(String name, Integer data) {
            this.name = name;
            this.size = data;
        }
    }
}

I need to calculate the Average of a field from a related list I have off of the Account record. I would like to either A) take this fields from the existing roll up summary field i have on the Account, OR B) Use the Aggregate function to calculate based off of the related list... 

I am not sure which is easier but if anyone can help me with this I would greatly appreciate it- this would be for a gauge chart. In the above code I am getting an error stating the constructor in line 16 is not defined- Highlighted in Bold