added info about using dashboard variables, resolves #9

content/v2.0/visualize-data/variables/_index.md

@@ -17,21 +17,47 @@ without having to edit the queries, making it easy to interact with your dashboa
 Variables are scoped by organization.
 
 ## Use dashboard variables
-- Variables are stored in a `v` object associated with each dashboard.
-  Variables are references using dot-notation (`v.variableName`).
-- In the Cell Editor and Data explorer, available variables are listed in the variables tab.
-  Must use the script builder. Click on a variable to input into your script.
-  Hover over the variable in the variables tab to select a value for the current query.
-  
+Both [custom dashboard variables](#manage-custom-variables) and [predefined dashboard variables](#predefined-dashboard-variables)
+are stored in a `v` object associated with each dashboard.
+Reference each variable using dot-notation (e.g. `v.variableName`).
 
-## Predefined dashboard variables
-
-##### v.timeRangeStart
-
-##### v.timeRangeStop
-
-##### v.windowPeriod
+```js
+from(bucket: v.bucket)
+  |> range(start: v.timeRangeStart, stop: v.timeRangeStart)
+  |> filter(fn (r) => r._measurement == v.measurement and r._field == v.field)
+  |> aggregateWindow(every: v.windowPeriod, fn: mean)
+```
 
 ## Manage custom variables
+As you build queries, In the Cell Editor and Data explorer, available variables are listed in the variables tab.
+Must use the script builder. Click on a variable to input into your script.
+Hover over the variable in the variables tab to select a value for the current query.
 
 {{< children >}}
+
+## Predefined dashboard variables
+The InfluxDB provides the following predefined dashboard variables:
+
+#### v.timeRangeStart
+Specifies the beginning of the queried time range.
+This variable is typically used to define the [`start` parameter](/v2.0/reference/flux/functions/built-in/transformations/range#start)
+of the `range()` function.
+
+The **Time Range** selector defines the value of this variable.
+
+#### v.timeRangeStop
+Specifies the end of the queried time range.
+This variable is typically used to define the [`stop` parameter](/v2.0/reference/flux/functions/built-in/transformations/range#stop)
+of the `range()` function.
+
+The **Time Range** selector defines the value of this variable.
+It defaults to `now`.
+
+#### v.windowPeriod
+Specifies the period of windowed data.
+This variable is typically used to define the `every` or `period` parameter of the
+[`window()` function](/v2.0/reference/flux/functions/built-in/transformations/window)
+in data aggregation operations.
+
+The value of this variable is calculated by dividing the total time within the displayed
+time range by the dashboard refresh interval (defined by the **Refresh** dropdown).