[SalesForce] How to use SUM and GROUP BY in SOQL

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 do a sub-select, e.g:

Select Amount,CloseDate,Name, (Select PricebookEntry.Product2Id, TotalPrice, UnitPrice, ListPrice From OpportunityLineItems) From Opportunity WHERE StageName = 'Closed' ORDER BY Amount DESC NULLS LAST

Opportunity Products (OpportunityLineItem) is a Detail to the Master Opportunity, i.e. 1:n between Opportunity and Product lines. Products belong to a Pricebook, thus you have to traverse the PricebookEntry relationship to retrieve the Product ID (if you need it).

You can also query having OpportunityLineItem as the Primary object and referencing Opportuinty via the Opportunity relationship:

Select Select PricebookEntry.Product2Id, TotalPrice, Opportunity.Amount, Opportunity.CloseDate, Opportunity.Name From OpportunityLineItem WHERE Opportunity.StageName = 'Closed' ORDER BY Opportunity.Amount

You can't go directly from Product2 to Opportunity because there is no direct relationship between the two, i.e. the relationship is via PricebookEntry.

A useful tool in discovering relationships between objects is to use the Force.com Eclipse plugin. You can double click on the salesforce.schema item and it will open a GUI for building queries: