added nonNegativeDerivative function

src/app/services/graphite/gfunc.js

@@ -142,6 +142,13 @@ function (_) {
     category: categories.Transform,
   });
 
+  addFuncDef({
+    name: 'nonNegativeDerivative',
+    category: categories.Transform,
+    params: [ { name: "max value or 0", type: "int", } ],
+    defaultParams: [0]
+  });
+
   addFuncDef({
     name: 'timeShift',
     category: categories.Transform,