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
@@ -2097,6 +2097,7 @@
     'PhabricatorAccessLog' => 'infrastructure/log/PhabricatorAccessLog.php',
     'PhabricatorAccessLogConfigOptions' => 'applications/config/option/PhabricatorAccessLogConfigOptions.php',
     'PhabricatorAccessibilitySetting' => 'applications/settings/setting/PhabricatorAccessibilitySetting.php',
+    'PhabricatorAccumulateChartFunction' => 'applications/fact/chart/PhabricatorAccumulateChartFunction.php',
     'PhabricatorActionListView' => 'view/layout/PhabricatorActionListView.php',
     'PhabricatorActionView' => 'view/layout/PhabricatorActionView.php',
     'PhabricatorActivitySettingsPanel' => 'applications/settings/panel/PhabricatorActivitySettingsPanel.php',
@@ -7961,6 +7962,7 @@
     'PhabricatorAccessLog' => 'Phobject',
     'PhabricatorAccessLogConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorAccessibilitySetting' => 'PhabricatorSelectSetting',
+    'PhabricatorAccumulateChartFunction' => 'PhabricatorChartFunction',
     'PhabricatorActionListView' => 'AphrontTagView',
     'PhabricatorActionView' => 'AphrontView',
     'PhabricatorActivitySettingsPanel' => 'PhabricatorSettingsPanel',
diff --git a/src/applications/fact/chart/PhabricatorAccumulateChartFunction.php b/src/applications/fact/chart/PhabricatorAccumulateChartFunction.php
new file mode 100644
--- /dev/null
+++ b/src/applications/fact/chart/PhabricatorAccumulateChartFunction.php
@@ -0,0 +1,37 @@
+<?php
+
+final class PhabricatorAccumulateChartFunction
+  extends PhabricatorChartFunction {
+
+  const FUNCTIONKEY = 'accumulate';
+
+  protected function newArguments() {
+    return array(
+      $this->newArgument()
+        ->setName('x')
+        ->setType('function')
+        ->setFunctionType('impulse')
+        ->setIsSourceFunction(true),
+    );
+  }
+
+  protected function newDatapoints(PhabricatorChartDataQuery $query) {
+    $source_function = $this->getSourceFunction();
+
+    // Use an unconstrained query to pull all the data from the underlying
+    // source. We need to accumulate data since the beginning of time to
+    // figure out the right Y-intercept -- otherwise, we'll always start at
+    // "0" wherever our domain begins.
+    $empty_query = new PhabricatorChartDataQuery();
+    $points = $source_function->getDatapoints($empty_query);
+
+    $accumulator = 0;
+    foreach ($points as $key => $point) {
+      $accumulator += $point['y'];
+      $points[$key]['y'] = $accumulator;
+    }
+
+    return $query->selectDatapoints($points);
+  }
+
+}
diff --git a/src/applications/fact/chart/PhabricatorChartDataQuery.php b/src/applications/fact/chart/PhabricatorChartDataQuery.php
--- a/src/applications/fact/chart/PhabricatorChartDataQuery.php
+++ b/src/applications/fact/chart/PhabricatorChartDataQuery.php
@@ -34,4 +34,43 @@
     return $this->limit;
   }
 
+  public function selectDatapoints(array $points) {
+    $x_min = $this->getMinimumValue();
+    $x_max = $this->getMaximumValue();
+    $limit = $this->getLimit();
+
+    if ($x_min !== null) {
+      foreach ($points as $key => $point) {
+        if ($point['x'] < $x_min) {
+          unset($points[$key]);
+        }
+      }
+    }
+
+    if ($x_max !== null) {
+      foreach ($points as $key => $point) {
+        if ($point['x'] > $x_max) {
+          unset($points[$key]);
+        }
+      }
+    }
+
+    // If we have too many data points, throw away some of the data.
+    if ($limit !== null) {
+      $count = count($points);
+      if ($count > $limit) {
+        $ii = 0;
+        $every = ceil($count / $limit);
+        foreach ($points as $key => $point) {
+          $ii++;
+          if (($ii % $every) && ($ii != $count)) {
+            unset($points[$key]);
+          }
+        }
+      }
+    }
+
+    return $points;
+  }
+
 }
diff --git a/src/applications/fact/chart/PhabricatorChartFunction.php b/src/applications/fact/chart/PhabricatorChartFunction.php
--- a/src/applications/fact/chart/PhabricatorChartFunction.php
+++ b/src/applications/fact/chart/PhabricatorChartFunction.php
@@ -73,6 +73,27 @@
     return $this->argumentParser;
   }
 
+  public function selectAllFunctions() {
+    $result = array();
+    $result[] = $this;
+
+    $parser = $this->getArgumentParser();
+    foreach ($parser->getAllArguments() as $argument) {
+      if ($argument->getType() !== 'function') {
+        continue;
+      }
+
+      $name = $argument->getName();
+      $value = $this->getArgument($name);
+
+      foreach ($value->selectAllFunctions() as $subfunction) {
+        $result[] = $subfunction;
+      }
+    }
+
+    return $result;
+  }
+
   public function loadData() {
     return;
   }
@@ -117,6 +138,34 @@
       return false;
     }
 
+    if ($this->isImpulseFunction()) {
+      return true;
+    }
+
+    $source_function = $this->getSourceFunction();
+    if ($source_function) {
+      return $source_function->hasDomain();
+    }
+
+    throw new PhutilMethodNotImplementedException();
+  }
+
+  public function isImpulseFunction() {
+    return false;
+  }
+
+  public function getDomain() {
+    // TODO: We can examine the data to fit a better domain.
+    if ($this->isImpulseFunction()) {
+      $now = PhabricatorTime::getNow();
+      return array($now - phutil_units('90 days in seconds'), $now);
+    }
+
+    $source_function = $this->getSourceFunction();
+    if ($source_function) {
+      return $source_function->getDomain();
+    }
+
     throw new PhutilMethodNotImplementedException();
   }
 
diff --git a/src/applications/fact/chart/PhabricatorChartFunctionArgument.php b/src/applications/fact/chart/PhabricatorChartFunctionArgument.php
--- a/src/applications/fact/chart/PhabricatorChartFunctionArgument.php
+++ b/src/applications/fact/chart/PhabricatorChartFunctionArgument.php
@@ -6,6 +6,7 @@
   private $name;
   private $type;
   private $isSourceFunction;
+  private $functionType;
 
   public function setName($name) {
     $this->name = $name;
@@ -49,6 +50,15 @@
     return $this->isSourceFunction;
   }
 
+  public function setFunctionType($function_type) {
+    $this->functionType = $function_type;
+    return $this;
+  }
+
+  public function getFunctionType() {
+    return $this->functionType;
+  }
+
   public function newValue($value) {
     switch ($this->getType()) {
       case 'fact-key':
@@ -119,8 +129,24 @@
               implode(', ', array_keys($functions))));
         }
 
-        return id(clone $functions[$function_name])
+        $function = id(clone $functions[$function_name])
           ->setArguments($value);
+
+        switch ($this->getFunctionType()) {
+          case 'impulse':
+            if (!$function->isImpulseFunction()) {
+              throw new Exception(
+                pht(
+                  'Function must be an impulse function, but "%s" is not '.
+                  'an impulse function.',
+                  $function_name));
+            }
+            break;
+          default:
+            break;
+        }
+
+        return $function;
       case 'number':
         if (!is_float($value) && !is_int($value)) {
           throw new Exception(
diff --git a/src/applications/fact/chart/PhabricatorChartFunctionArgumentParser.php b/src/applications/fact/chart/PhabricatorChartFunctionArgumentParser.php
--- a/src/applications/fact/chart/PhabricatorChartFunctionArgumentParser.php
+++ b/src/applications/fact/chart/PhabricatorChartFunctionArgumentParser.php
@@ -72,6 +72,10 @@
     return $this;
   }
 
+  public function getAllArguments() {
+    return array_values($this->argumentMap);
+  }
+
   public function parseArguments() {
     $have_count = count($this->rawArguments);
     $want_count = count($this->argumentMap);
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
@@ -22,6 +22,10 @@
     return $fact->getFunctionArguments();
   }
 
+  public function isImpulseFunction() {
+    return true;
+  }
+
   public function loadData() {
     $fact = $this->fact;
 
@@ -44,73 +48,32 @@
       return;
     }
 
-    $points = array();
-
-    $sum = 0;
-    foreach ($data as $key => $row) {
-      $sum += (int)$row['value'];
-      $points[] = array(
-        'x' => (int)$row['epoch'],
-        'y' => $sum,
-      );
-    }
-
-    $this->datapoints = $points;
-  }
-
-  public function getDatapoints(PhabricatorChartDataQuery $query) {
-    $points = $this->datapoints;
-    if (!$points) {
-      return array();
-    }
-
-    $x_min = $query->getMinimumValue();
-    $x_max = $query->getMaximumValue();
-    $limit = $query->getLimit();
+    $map = array();
+    foreach ($data as $row) {
+      $value = (int)$row['value'];
+      $epoch = (int)$row['epoch'];
 
-    if ($x_min !== null) {
-      foreach ($points as $key => $point) {
-        if ($point['x'] < $x_min) {
-          unset($points[$key]);
-        }
+      if (!isset($map[$epoch])) {
+        $map[$epoch] = 0;
       }
-    }
 
-    if ($x_max !== null) {
-      foreach ($points as $key => $point) {
-        if ($point['x'] > $x_max) {
-          unset($points[$key]);
-        }
-      }
+      $map[$epoch] += $value;
     }
 
-    // If we have too many data points, throw away some of the data.
-    if ($limit !== null) {
-      $count = count($points);
-      if ($count > $limit) {
-        $ii = 0;
-        $every = ceil($count / $limit);
-        foreach ($points as $key => $point) {
-          $ii++;
-          if (($ii % $every) && ($ii != $count)) {
-            unset($points[$key]);
-          }
-        }
-      }
+    $points = array();
+    foreach ($map as $x => $y) {
+      $points[] = array(
+        'x' => $x,
+        'y' => $y,
+      );
     }
 
-    return $points;
+    $this->datapoints = $points;
   }
 
-  public function hasDomain() {
-    return true;
+  protected function newDatapoints(PhabricatorChartDataQuery $query) {
+    return $query->selectDatapoints($this->datapoints);
   }
 
-  public function getDomain() {
-    // TODO: We can examine the data to fit a better domain.
-
-    $now = PhabricatorTime::getNow();
-    return array($now - phutil_units('90 days in seconds'), $now);
-  }
 
 }
diff --git a/src/applications/fact/chart/PhabricatorShiftChartFunction.php b/src/applications/fact/chart/PhabricatorShiftChartFunction.php
--- a/src/applications/fact/chart/PhabricatorShiftChartFunction.php
+++ b/src/applications/fact/chart/PhabricatorShiftChartFunction.php
@@ -22,7 +22,7 @@
   }
 
   protected function evaluateFunction($x) {
-    return $x * $this->getArgument('shift');
+    return $x + $this->getArgument('shift');
   }
 
 }
diff --git a/src/applications/fact/controller/PhabricatorFactChartController.php b/src/applications/fact/controller/PhabricatorFactChartController.php
--- a/src/applications/fact/controller/PhabricatorFactChartController.php
+++ b/src/applications/fact/controller/PhabricatorFactChartController.php
@@ -14,12 +14,6 @@
 
     $functions = array();
 
-    $functions[] = id(new PhabricatorFactChartFunction())
-      ->setArguments(array('tasks.count.create'));
-
-    $functions[] = id(new PhabricatorFactChartFunction())
-      ->setArguments(array('tasks.open-count.create'));
-
     $x_function = id(new PhabricatorXChartFunction())
       ->setArguments(array());
 
@@ -42,12 +36,21 @@
               array(
                 'scale',
                 array('x'),
-                0.001,
+                0.0001,
               ),
             ),
-            10,
+            200,
+          ),
+          75,
+        ));
+
+    $functions[] = id(new PhabricatorAccumulateChartFunction())
+      ->setArguments(
+        array(
+          array(
+            'fact',
+            'tasks.count.create',
           ),
-          200,
         ));
 
     list($domain_min, $domain_max) = $this->getDomain($functions);
@@ -61,6 +64,12 @@
       ->setMaximumValue($domain_max)
       ->setLimit(2000);
 
+    foreach ($functions as $function) {
+      foreach ($function->selectAllFunctions() as $subfunction) {
+        $subfunction->loadData();
+      }
+    }
+
     $datasets = array();
     foreach ($functions as $function) {
       $function->setXAxis($axis);