Charts: update the 'Load Data On Demand' demo description (DevExpress…

…#1896)

JSDemos/Demos/Charts/LoadDataOnDemand/description.md

@@ -1 +1,27 @@
-In this demo, the chart loads data as you pan it. Once a data block is loaded, it stays in memory to reduce requests to the server.
+In this demo, the [range area](https://js.devexpress.com/Demos/WidgetsGallery/Demo/Charts/RangeArea/) Chart loads data as you pan it. Once the component loads a data block, this data stays in memory to reduce requests to the server. To implement this functionality, do the following:
+
+1. Configure a data source.
+
+    - Assign a [DataSource](/Documentation/ApiReference/Data_Layer/DataSource/) with an empty [store](/Documentation/ApiReference/Data_Layer/DataSource/Configuration/store/) to the Chart [dataSource](/Documentation/ApiReference/UI_Components/dxChart/Configuration/#dataSource) property.
+
+    - Set the [sort](/Documentation/ApiReference/Data_Layer/DataSource/Configuration/#sort) property to `date`.
+
+    - Disable the [paginate](/Documentation/ApiReference/Data_Layer/DataSource/Configuration/#paginate) property to prevent data from partitioning.
+
+2. Configure the Chart to support on-demand data loading.
+
+    - Set the [visualRangeUpdateMode](/Documentation/ApiReference/UI_Components/dxChart/Configuration/argumentAxis/#visualRangeUpdateMode) property to `keep`.
+
+    - Specify a [loadingIndicator](/Documentation/ApiReference/UI_Components/dxChart/Configuration/loadingIndicator/).
+
+    - Specify the initial [visualRange](/Documentation/ApiReference/UI_Components/dxChart/Configuration/argumentAxis/visualRange/).
+
+    - Implement a function that respond to user pan operations and tracks [visualRange](/Documentation/ApiReference/UI_Components/dxChart/Configuration/argumentAxis/visualRange/) changes. If a user changed the visual range, initiate a request for additional data.
+
+3. Implement functions that load the data.
+
+    - In this demo, Chart displays daily information. If a user pan the Chart left or right more than half a day, additional data starts to load. The `onVisualRangeChanged` initiates this procedure.
+
+    - The `uploadDataByVisualRange` function analyzes the starting and ending points of the visual range and the bounds of already loaded data. If necessary, it calls the `getDataFrame` function to obtain new data points. Finally, it reloads the [DataSource](/Documentation/ApiReference/Data_Layer/DataSource/) and saves the current visual range.
+
+    - The `getDataFrame` Ajax request gets the new data frame from the server.