Update description of vz-line-chart to add the 'relative' property.

Update description of the DEVELOPMENT.md file: bower and gulp are actually
required to be installed globally.
Change: 128423812

Author: RenatoUtsch

tensorflow/tensorboard/DEVELOPMENT.md

@@ -9,8 +9,8 @@ nodejs-legacy npm`.
 
 Next, you'll want to install [gulp](http://gulpjs.com/) and
 [bower](http://bower.io/), which are used for build tooling and dependency
-management respectively. `sudo npm install -g gulp bower` will install them
-globally for convenience.
+management respectively. Both must be installed globally: `sudo npm install -g
+gulp bower` will do that.
 
 Then, cd into the TensorBoard directory:
 

tensorflow/tensorboard/components/vz-line-chart/vz-line-chart.html

@@ -141,8 +141,10 @@
         /**
          * The way to display the X values. Allows:
          * - "step" - Linear scale using the  "step" property of the datum.
-         * - "wall_time" - Temoral scale using the "wall_time" property of the
+         * - "wall_time" - Temporal scale using the "wall_time" property of the
          * datum.
+         * - "relative" - Temporal scale using the "relative" property of the
+         * datum if it is present or calculating from "wall_time" if it isn't.
          */
         xType: {
           type: String,