[SalesForce] Compile Error: Invalid Type

This is untested so you may need to have play, however this should return a List of the RevenueWrapper classes which contains the 4 columns you are looking to use in your VF page:

public class RevenueController{

Map<String, RevenueWrapper> ResultList = new Map<String, RevenueWrapper>(); 

public RevenueController(){
    List<AggregateResult> result =  [
    SELECT FISCAL_YEAR(Date__c) Date, Type__c, sum(Amount__c) Amount
    FROM Revenue__c
    GROUP BY FISCAL_YEAR(Date__c), Type__c];

    for(AggregateResult ar : result){

        RevenueWrapper RW = new RevenueWrapper();

        //If the RevenueWrapper already exists grab it from the Map
        if(ResultList.containsKey(String.valueOf(ar.get('Date')))){
            RW = ResultList.get(String.valueOf(ar.get('Date')));
        }

        //Set the amount
        RW.setAmount(ar);

        //Save back to the Map
        ResultList.put(String.valueOf(ar.get('Date')), RW);
    }
}

public List<RevenueWrapper> getResults() {
    return ResultList.values();
}


public class RevenueWrapper{
    public String revenueDate { get; set; }
    public Integer budgetAmount { get; set; }
    public Integer actualAmount { get; set; }
    public Integer varianceFYPercent { set; }
    public Integer varianceFYAmount { set; }

    public RevenueWrapper(){

    }

    public void setAmount(AggregateResult ar){
        revenueDate = String.valueOf(ar.get('Date'));

            //Set the amount value based on type
        if(ar.get('Type__c') == 'Budget'){
            budgetAmount = Integer.valueOf(ar.get('Amount'));
        }else if(ar.get('Type__c') == 'Actual'){
            actualAmount = Integer.valueOf(ar.get('Amount'));
        }else{ //Catch all for a type that isn't recognised
            return;
        }

    }

    public Integer getVarianceFYAmount(){
        return actualAmount - budgetAmount;
    }

    public Decimal getVarianceFYPercent(){
        if(budgetAmount != 0){
            return Decimal.valueOf((Double.valueOf(actualAmount) / Double.valueOf(budgetAmount)) * 100).setscale(1);
        } else{
            return 0;
        }
    }
}

}

You can't instantiate a List like that using an integer. You don't need to explicitly specify how much items are going into the list when it is created. Instead, just remove the integer:

//Instantiate the list
String[] myArray = new List<String>();

You're logic in your loop seems to be incorrect as well. Specifically, myArray's size will always be 0 when you instantiate it. Change your loop logic to:

for(Integer i=0;i<length;i++) {
   // code
}

You want to generate a List of strings based on the i variable. You will want to change your internal logic to:

//Populate the array
myArray.add('Test ' + i);

// Write value to the debug log
System.debug(myArray[i]);

Finally, you want to return that Array. You need to change your method so the return type is no longer void by changing its signature:

public static String[] generateStringArray(Integer length)

then it is just a matter of returning that array at the end of your method:

return myArray;

This would bring it all together as:

public class StringArrayTest {
    //Public Method
    public static String[] generateStringArray(Integer length) {
        //Instantiate the list
        String[] myArray = new List<String>();

        //Iterate throught the list
        for(Integer i=0;i<length;i++) {
            //Populate the array
            myArray.add('Test ' + i);

            // Write value to the debug log
            System.debug(myArray[i]);
        } //end loop

        return myArray;
    }//end method       
}// end class

Some more info on Arrays and Lists, Loops, and Class methods for future reference.