Forgot to add files in previous commit and document two more pieces of code.

core/Piwik.php

@@ -116,7 +116,12 @@ class Piwik
     }
 
     /**
-     * TODO
+     * Safely compute a ratio. Returns 0 if divisor is 0 (to avoid division by 0 error).
+     *
+     * @param number $dividend
+     * @param number $divisor
+     * @param int $precision
+     * @return number
      */
     public static function getQuotientSafe($dividend, $divisor, $precision = 0)
     {

core/Plugin/AggregatedMetric.php

@@ -10,9 +10,11 @@ namespace Piwik\Plugin;
 use Piwik\DataTable\Row;
 
 /**
- * TODO
+ * Base type for metric metadata classes that describe aggregated metrics. These metrics are
+ * computed in the backend data store and are aggregated in PHP when Piwik archives period reports.
  *
- * TODO: note that this will be filled out in another issue
+ * Note: This class is a placeholder. It will be filled out at a later date. Right now, only
+ * processed metrics can be defined this way.
  */
 abstract class AggregatedMetric extends Metric
 {

plugins/MultiSites/Metrics/RevenueEvolution.php

+<?php
+/**
+ * Piwik - free/libre analytics platform
+ *
+ * @link http://piwik.org
+ * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
+ */
+
+namespace Piwik\Plugins\MultiSites\Metrics;
+
+use Piwik\DataTable\Row;
+use Piwik\Plugins\CoreHome\Metrics\EvolutionMetric;
+use Piwik\Site;
+
+/**
+ * TODO
+ */
+class RevenueEvolution extends EvolutionMetric
+{
+    public function compute(Row $row)
+    {
+        $columnName = $this->getWrappedName();
+        $currentValue = $this->getMetric($row, $columnName);
+
+        // if the site this is for doesn't support ecommerce & this is for the revenue_evolution column,
+        // we don't add the new column
+        if ($currentValue === false
+            && !Site::isEcommerceEnabledFor($row->getColumn('label'))
+        ) {
+            return false;
+        }
+
+        return parent::compute($row);
+    }
+}
\ No newline at end of file