Documentation for history_graph (#3414)

* Add custom ui documentation

* Documentation for history_graph

* history_graph is due for 0.55

source/_components/history_graph.markdown

@@ -0,0 +1,62 @@
+---
+layout: page
+title: "History Graph"
+description: "Instructions for setting up History Graph."
+date: 2017-09-20 15:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: History
+logo: home-assistant.png
+ha_release: "0.55"
+---
+<p class='img'>
+  <img src='{{site_root}}/images/screenshots/history_graph.png' />
+</p>
+
+The `history_graph` component will make the UI display a graph similar to the graphs in `more-info` popups and the [history](/components/history/) panel.
+
+To use this component in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Minimal configuration.yaml entry
+history_graph:
+  gr1:
+    entities:
+      - light.ceiling.lights
+  
+```
+
+### {% linkable_title Configuration variables %}
+
+| Attribute                 | Optional | Default | Description                                 |
+|---------------------------|----------|-------------------------------------------------------|
+| `entities`                |   no     |         | List of entities whose history to show as a graph.  |
+| `name`                    |   yes    |  ID     | Name to display. |
+| `hours_to_show`           |   yes    |  24     | Number of hours to show. |
+| `refresh`                 |   yes    |   0     | Number of seconds between graph refreshes. 0 for no refreshes. |
+
+### {% linkable_title Full Example %}
+
+```yaml
+# Full configuration.yaml entry
+history_graph:
+  gr1:
+    name: Lights Graph
+    entities:
+      - light.ceiling.lights
+      - light.bed_light
+    hours_to_show: 240
+    refresh: 60
+  gr2:
+    name: Temperature
+    entities:
+      - sensor.outside_temperature
+      - sensor.inside_temperature
+    hours_to_show: 120
+    # refresh: 0
+
+```
+
+