diff --git a/render.js b/render.js
index 5a1f6b8..1d8a407 100644
--- a/render.js
+++ b/render.js
@@ -974,6 +974,61 @@
   }
 
 
+  function deltaBy(ingrid, keys) {
+    var outgrid = {headers: ingrid.headers, data: [], types: ingrid.types};
+    for (var rowi = 0; rowi < ingrid.data.length; rowi++) {
+      var row = ingrid.data[rowi];
+      outgrid.data.push(row);
+    }
+
+    var keycols = [];
+    for (var keyi in keys) {
+      var key = keys[keyi];
+      keycols.push(keyToColNum(ingrid, key));
+    }
+
+    if (outgrid.data.length < 2) {
+      return outgrid;
+    }
+    for (var keyi in keycols) {
+      var keycol = keycols[keyi];
+
+      var prev_val = undefined;
+      for (var rowi = 1; rowi < outgrid.data.length; rowi++) {
+        var row = outgrid.data[rowi];
+        var val = row[keycol];
+        console.debug('prev_val: ', prev_val, ' val: ', val);
+        if (val == undefined) {
+          continue;
+        } else if (outgrid.types[keycol] === T_NUM) {
+          if (prev_val != undefined) {
+            if (val > prev_val) {
+              var new_val = val - prev_val;
+              console.debug('out: ', new_val);
+              outgrid.data[rowi][keycol] = new_val;
+            } else if (val == prev_val) {
+              console.debug('out: ', undefined);
+              outgrid.data[rowi][keycol] = undefined;
+            }
+          }
+          prev_val = val;
+        }
+      }
+    }
+
+    return outgrid;
+  }
+
+
+  function doDeltaBy(grid, argval) {
+    console.debug('deltaBy:', argval);
+    console.debug('grid:', grid);
+    grid = deltaBy(grid, argval.split(','));
+    console.debug('grid:', grid);
+    return grid;
+  }
+
+
   function orderBy(grid, keys) {
     var keycols = [];
     for (var keyi in keys) {
@@ -1229,6 +1284,8 @@
         transform(doQueryBy, argval);
       } else if (argkey == 'limit') {
         transform(doLimit, argval);
+      } else if (argkey == 'delta') {
+        transform(doDeltaBy, argval);
       } else if (argkey == 'order') {
         transform(doOrderBy, argval);
       } else if (argkey == 'extract_regexp') {
@@ -1627,6 +1684,7 @@
       stringifiedCols: stringifiedCols,
       filterBy: filterBy,
       queryBy: queryBy,
+      deltaBy: deltaBy,
       orderBy: orderBy,
       extractRegexp: extractRegexp,
       fillNullsWithZero: fillNullsWithZero,
diff --git a/t/trender.js b/t/trender.js
index cfac92e..ba09458 100644
--- a/t/trender.js
+++ b/t/trender.js
@@ -109,6 +109,35 @@
 });
 
 
+wvtest('delta', function() {
+  var grid = {
+    headers: ['a', 'b'],
+    types: ['number', 'number'],
+    data: [
+      [0, 1],
+      [5, 7],
+      [30, 1],
+      [2, 1],
+      [2, 1],
+    ]
+  };
+  var dt = afterquery.internal.deltaBy(grid, 'a');
+  dump(dt);
+  WVPASSEQ(dt.data.length, 5);
+  WVPASSEQ(dt.data[0][0], 0);
+  WVPASSEQ(dt.data[0][1], 1);
+  WVPASSEQ(dt.data[1][0], 5);
+  WVPASSEQ(dt.data[1][1], 7);
+  WVPASSEQ(dt.data[2][0], 25);
+  WVPASSEQ(dt.data[2][1], 1);
+  WVPASSEQ(dt.data[3][0], 2);
+  WVPASSEQ(dt.data[3][1], 1);
+  WVPASSEQ(dt.data[4][0], undefined);
+  WVPASSEQ(dt.data[4][1], 1);
+});
+
+
+
 wvtest('guessTypes', function() {
   var data1 = [['1999-01-01', '1999-02-02', 1, 2.5, false, 'foo']];
   var data2 = [['1999-01-01', '1999-02-02 12:34', 2, 'x', true, null]];