Updated readme for new column chart

CHANGELOG.md

@@ -1,4 +1,4 @@
-## 1.4.1
+## 1.4.1 [unreleased]
 
 - Added new bar chart format
 

README.md

@@ -254,12 +254,20 @@ SELECT date_trunc('week', created_at), gender, COUNT(*) FROM users GROUP BY 1, 2
 
 ### Column Chart
 
+There are also two ways to generate column charts.
+
 2+ columns - string, numeric(s) - [Example](https://blazerme.herokuapp.com/queries/2-top-genres)
 
 ```sql
 SELECT gender, COUNT(*) FROM users GROUP BY 1
 ```
 
+3 columns - string, string, numeric [master]
+
+```sql
+SELECT gender, zip_code, COUNT(*) FROM users GROUP BY 1, 2
+```
+
 ### Maps
 
 Columns named `latitude` and `longitude` or `lat` and `lon` - [Example](https://blazerme.herokuapp.com/queries/11-airports-in-pacific-time-zone)