Updated docs

diff --git a/docs/07-Advanced.md b/docs/07-Advanced.md
index d040d4584ac20a866b415ff2112cec08ab19c7c0..8bc94cc1f05719702fe84fc9eb30cc73230410e1 100644 (file)
--- a/docs/07-Advanced.md
+++ b/docs/07-Advanced.md
@@ -360,6 +360,9 @@ The built in controller types are:
 * `Chart.controllers.polarArea`
 * `Chart.controllers.bubble`
 
+#### Bar Controller
+The bar controller has a special property that you should be aware of. To correctly calculate the width of a bar, the controller must determine the number of datasets that map to bars. To do this, the bar controller attaches a property `bar` to the dataset during initialization. If you are creating a replacement or updated bar controller, you should do the same. This will ensure that charts with regular bars and your new derived bars will work seamlessly.
+
 ### Building Chart.js
 
 Chart.js uses <a href="http://gulpjs.com/" target="_blank">gulp</a> to build the library into a single JavaScript file.