Commented Groovy Example

// get the look-back perios - number of months to look back
// utilize the Elvis operator - in case the user entry is null or zero, lookback will be 12
def lookback = api.decimalUserEntry('Look-back Period [months]') ?: 12

// create the look-back date by utilizing Joda DateTime and subtracting the look-back period from the current date
def lookbackDate = new DateTime().plusMonths(-lookback.toInteger()).toDate()

// get all competitor records from the Product Extension whose validity is greater or equal to the look-back date
// we need to convert the date to its String representation because in the PX all attributes are stored as Strings
def competitors = api.productExtension('Competition', Filter.greaterOrEqual('attribute2', lookbackDate.format('yyyy-MM-dd')))

// the competitors will evaluate to true if it's a non-null and non-empty collection
if (competitors) {
  
  // sort the competitors by the date (ascending) first and price (ascending) second using a custom comparator
  // use a Groovy Closure for that - a, b represent the two objects being compared
  def sortedCompetitors = competitors.sort{ a, b -> 
	
    // the dates are stored as Strings in format 'yyyy-MM-dd' so it's safe to compare them lexicographically
    def date1 = a.attribute2
    def date2 = b.attribute2
    
    // we need to convert the prices from String to BigDecimal for it to be compared properly
    // the save navigation operator ?. will evaluate the entire expression to null if attribute3 is null, preventing a Null Pointer Exception
    def price1 = a.attribute3?.toBigDecimal()
    def price2 = b.attribute3?.toBigDecimal()

	// the Spaceship operator delegates to the compareTo method of its operands
    // notice how we combine it with the Elvis operator
    // if the comparison of the dates is zero (equal dates), which evaluates to false, it will compare the prices
    return date1 <=> date2 ?: price1?.toBigDecimal() <=> price2?.toBigDecimal()
  }
  
  // here we convert the list of competitors sorted by date and price to a map where the competitor name is the key and the competitor record is the value
  // the result will be a map where each competitor will have the latest price
  // that's because we sorted the list earlier in ascending order, so each record with later date or higher price overwrites the previous one
  def latestPricePerCompetitor = sortedCompetitors.collectEntries {
	
    // the left side will evaluate to the String representation thanks to the parentheses
    // if you don't explicitly name the iterator in the Closure, you can reference it by 'it'
    [ (it.attribute1): it ]
  }  

  // we now create a matrix which will have three columns
  def columns = [ 'Competitor', 'Date', 'Price' ]

  // since the method api.newMatrix doesn't accept a List but an array of Strings, we use the Spread Operator to unwrap each element of the List and pass it to the method
  // (of course we could type in the column names directly but we needed to demonstrate the Spread operator)
  def matrix = api.newMatrix(*columns)

  // again, we use a Closure to iterate through each element of the map
  // but this time we rename the default 'it' to 'c' to demonstrate this feature
  latestPricePerCompetitor.each{ name, record ->
	
    // we add a row to the matrix for each entry in the map
    matrix.addRow([
      'Competitor' : name,
      'Date' : record.attribute2,
      'Price' : record.attribute3
    ])
  }

  // return the matrix
  return matrix
}
return null