diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2669,6 +2669,7 @@
     'PhabricatorChartFunctionArgument' => 'applications/fact/chart/PhabricatorChartFunctionArgument.php',
     'PhabricatorChartFunctionArgumentParser' => 'applications/fact/chart/PhabricatorChartFunctionArgumentParser.php',
     'PhabricatorChartFunctionLabel' => 'applications/fact/chart/PhabricatorChartFunctionLabel.php',
+    'PhabricatorChartInterval' => 'applications/fact/chart/PhabricatorChartInterval.php',
     'PhabricatorChartRenderingEngine' => 'applications/fact/engine/PhabricatorChartRenderingEngine.php',
     'PhabricatorChartStackedAreaDataset' => 'applications/fact/chart/PhabricatorChartStackedAreaDataset.php',
     'PhabricatorChatLogApplication' => 'applications/chatlog/application/PhabricatorChatLogApplication.php',
@@ -8685,6 +8686,7 @@
     'PhabricatorChartFunctionArgument' => 'Phobject',
     'PhabricatorChartFunctionArgumentParser' => 'Phobject',
     'PhabricatorChartFunctionLabel' => 'Phobject',
+    'PhabricatorChartInterval' => 'Phobject',
     'PhabricatorChartRenderingEngine' => 'Phobject',
     'PhabricatorChartStackedAreaDataset' => 'PhabricatorChartDataset',
     'PhabricatorChatLogApplication' => 'PhabricatorApplication',
diff --git a/src/applications/fact/chart/PhabricatorChartInterval.php b/src/applications/fact/chart/PhabricatorChartInterval.php
new file mode 100644
--- /dev/null
+++ b/src/applications/fact/chart/PhabricatorChartInterval.php
@@ -0,0 +1,62 @@
+<?php
+
+final class PhabricatorChartInterval
+  extends Phobject {
+
+  private $min;
+  private $max;
+
+  public function __construct($min, $max) {
+    $this->min = $min;
+    $this->max = $max;
+  }
+
+  public static function newFromIntervalList(array $intervals) {
+    $min = null;
+    $max = null;
+    foreach ($intervals as $interval) {
+      if ($interval === null) {
+        continue;
+      }
+
+      $interval_min = $interval->getMin();
+      if ($interval_min !== null) {
+        if ($min === null) {
+          $min = $interval_min;
+        } else {
+          $min = min($min, $interval_min);
+        }
+      }
+
+      $interval_max = $interval->getMax();
+      if ($interval_max !== null) {
+        if ($max === null) {
+          $max = $interval_max;
+        } else {
+          $max = max($max, $interval_max);
+        }
+      }
+    }
+
+    return new self($min, $max);
+  }
+
+  public function setMin($min) {
+    $this->min = $min;
+    return $this;
+  }
+
+  public function getMin() {
+    return $this->min;
+  }
+
+  public function setMax($max) {
+    $this->max = $max;
+    return $this;
+  }
+
+  public function getMax() {
+    return $this->max;
+  }
+
+}
diff --git a/src/applications/fact/chart/PhabricatorFactChartFunction.php b/src/applications/fact/chart/PhabricatorFactChartFunction.php
--- a/src/applications/fact/chart/PhabricatorFactChartFunction.php
+++ b/src/applications/fact/chart/PhabricatorFactChartFunction.php
@@ -73,10 +73,10 @@
   }
 
   public function getDomain() {
-    return array(
-      head_key($this->map),
-      last_key($this->map),
-    );
+    $min = head_key($this->map);
+    $max = last_key($this->map);
+
+    return new PhabricatorChartInterval($min, $max);
   }
 
   public function newInputValues(PhabricatorChartDataQuery $query) {
diff --git a/src/applications/fact/chart/PhabricatorHigherOrderChartFunction.php b/src/applications/fact/chart/PhabricatorHigherOrderChartFunction.php
--- a/src/applications/fact/chart/PhabricatorHigherOrderChartFunction.php
+++ b/src/applications/fact/chart/PhabricatorHigherOrderChartFunction.php
@@ -4,37 +4,12 @@
   extends PhabricatorChartFunction {
 
   public function getDomain() {
-    $minv = array();
-    $maxv = array();
+    $domains = array();
     foreach ($this->getFunctionArguments() as $function) {
-      $domain = $function->getDomain();
-      if ($domain !== null) {
-        list($min, $max) = $domain;
-        if ($min !== null) {
-          $minv[] = $min;
-        }
-        if ($max !== null) {
-          $maxv[] = $max;
-        }
-      }
-    }
-
-    if (!$minv && !$maxv) {
-      return null;
-    }
-
-    $min = null;
-    $max = null;
-
-    if ($minv) {
-      $min = min($minv);
-    }
-
-    if ($maxv) {
-      $max = max($maxv);
+      $domains[] = $function->getDomain();
     }
 
-    return array($min, $max);
+    return PhabricatorChartInterval::newFromIntervalList($domains);
   }
 
   public function newInputValues(PhabricatorChartDataQuery $query) {
diff --git a/src/applications/fact/engine/PhabricatorChartRenderingEngine.php b/src/applications/fact/engine/PhabricatorChartRenderingEngine.php
--- a/src/applications/fact/engine/PhabricatorChartRenderingEngine.php
+++ b/src/applications/fact/engine/PhabricatorChartRenderingEngine.php
@@ -133,15 +133,15 @@
       $subfunction->loadData();
     }
 
-    list($domain_min, $domain_max) = $this->getDomain($functions);
+    $domain = $this->getDomain($functions);
 
     $axis = id(new PhabricatorChartAxis())
-      ->setMinimumValue($domain_min)
-      ->setMaximumValue($domain_max);
+      ->setMinimumValue($domain->getMin())
+      ->setMaximumValue($domain->getMax());
 
     $data_query = id(new PhabricatorChartDataQuery())
-      ->setMinimumValue($domain_min)
-      ->setMaximumValue($domain_max)
+      ->setMinimumValue($domain->getMin())
+      ->setMaximumValue($domain->getMax())
       ->setLimit(2000);
 
     $wire_datasets = array();
@@ -155,8 +155,8 @@
 
     $chart_data = array(
       'datasets' => $wire_datasets,
-      'xMin' => $domain_min,
-      'xMax' => $domain_max,
+      'xMin' => $domain->getMin(),
+      'xMax' => $domain->getMax(),
       'yMin' => $y_min,
       'yMax' => $y_max,
     );
@@ -165,46 +165,25 @@
   }
 
   private function getDomain(array $functions) {
-    $domain_min_list = null;
-    $domain_max_list = null;
-
+    $domains = array();
     foreach ($functions as $function) {
-      $domain = $function->getDomain();
-
-      list($function_min, $function_max) = $domain;
-
-      if ($function_min !== null) {
-        $domain_min_list[] = $function_min;
-      }
-
-      if ($function_max !== null) {
-        $domain_max_list[] = $function_max;
-      }
+      $domains[] = $function->getDomain();
     }
 
-    $domain_min = null;
-    $domain_max = null;
-
-    if ($domain_min_list) {
-      $domain_min = min($domain_min_list);
-    }
-
-    if ($domain_max_list) {
-      $domain_max = max($domain_max_list);
-    }
+    $domain = PhabricatorChartInterval::newFromIntervalList($domains);
 
     // If we don't have any domain data from the actual functions, pick a
     // plausible domain automatically.
 
-    if ($domain_max === null) {
-      $domain_max = PhabricatorTime::getNow();
+    if ($domain->getMax() === null) {
+      $domain->setMax(PhabricatorTime::getNow());
     }
 
-    if ($domain_min === null) {
-      $domain_min = $domain_max - phutil_units('365 days in seconds');
+    if ($domain->getMin() === null) {
+      $domain->setMin($domain->getMax() - phutil_units('365 days in seconds'));
     }
 
-    return array($domain_min, $domain_max);
+    return $domain;
   }
 
 }