Merge pull request #368 from influxdata/flux/difference-keep-first

Added keepFirst parameter to difference function

content/v2.0/reference/flux/functions/built-in/transformations/aggregates/difference.md

@@ -17,7 +17,11 @@ _**Function type:** Aggregate_
 _**Output data type:** Float_
 
 ```js
-difference(nonNegative: false, column: "_value")
+difference(
+  nonNegative: false,
+  column: "_value",
+  keepFirst: false
+)
 ```
 
 ## Parameters
@@ -34,6 +38,13 @@ Defaults to `"_value"`.
 
 _**Data type:** String_
 
+### keepFirst
+Indicates the first row should be kept.
+If `true`, the difference will be `null`.
+Defaults to `false`.
+
+_**Data type:** Boolean_
+
 ## Subtraction rules for numeric types
 - The difference between two non-null values is their algebraic difference;
   or `null`, if the result is negative and `nonNegative: true`;
@@ -90,6 +101,20 @@ from(bucket: "example-bucket")
 | 0004  | 6      | tv  |
 | 0005  | null   | tv  |
 
+
+#### With keepFirst set to true
+```js
+|> difference(nonNegative: false, keepfirst: true):
+```
+###### Output table
+| _time | _value | tag |
+|:-----:|:------:|:---:|
+| 0001  | null   | tv  |
+| 0002  | null   | tv  |
+| 0003  | -2     | tv  |
+| 0004  | 6      | tv  |
+| 0005  | null   | tv  |
+
 <hr style="margin-top:4rem"/>
 
 ##### Related InfluxQL functions and statements: