DOC: Add histogram category order example (#2779)

Author: hweecat

doc/python/histograms.md

@@ -20,7 +20,7 @@ jupyter:
     name: python
     nbconvert_exporter: python
     pygments_lexer: ipython3
-    version: 3.7.3
+    version: 3.8.5
   plotly:
     description: How to make Histograms in Python with Plotly.
     display_as: statistical
@@ -30,9 +30,9 @@ jupyter:
     order: 3
     page_type: example_index
     permalink: python/histograms/
-    redirect_from: 
-      - /python/histogram-tutorial/
-      - /python/histogram/
+    redirect_from:
+    - /python/histogram-tutorial/
+    - /python/histogram/
     thumbnail: thumbnail/histogram.jpg
 ---
 
@@ -392,6 +392,21 @@ fig.update_layout(
 fig.show()
 ```
 
+### Sort Histogram by Category Order
+
+Histogram bars can also be sorted based on the ordering logic of the categorical values using the [categoryorder](https://plotly.com/python/reference/layout/xaxis/#layout-xaxis-categoryorder) attribute of the x-axis. Sorting of histogram bars using `categoryorder` also works with multiple traces on the same x-axis. In the following examples, the histogram bars are sorted based on the total numerical values.
+
+```python
+import plotly.express as px
+import pandas as pd
+
+df = px.data.tips()
+fig1 = px.histogram(df, x="day").update_xaxes(categoryorder='total ascending')
+fig2 = px.histogram(df, x="day", color="smoker").update_xaxes(categoryorder='total descending')
+fig1.show()
+fig2.show()
+```
+
 #### Reference
 
 See https://plotly.com/python/reference/histogram/ for more information and chart attribute options!