Repository: pyjanitor-devs/pyjanitor

Issue:
Example of groupby_agg shows wrong output
# Brief Description of Fix

<!-- Please describe the fix in terms of a "before" and "after". In other words, what's not so good about the current docs
page, and what you would like to see it become.

Example starter wording is provided. -->

Currently, the docs for `groupby_agg` uses `new_column_name='count'` but the result shown in the docs has column name 'size'.

<img width="554" alt="image" src="https://user-images.githubusercontent.com/30731072/146632174-9b4a1022-d205-4ead-855e-76a3dccb7b7a.png">

The correct column name should be 'count'.

<img width="330" alt="image" src="https://user-images.githubusercontent.com/30731072/146632197-7607612a-8388-4383-846b-07ac3d7b522d.png">

```
   group  var1  count
0      1     1      4
1      1     1      4
2      1     1      4
3      1     1      4
4      1     2      1
5      2     1      1
6      2     2      3
7      2     2      3
8      2     2      3
9      2     3      1
```

Seems like a simple enough fix, I'm happy to open a PR if you'ld like

# Relevant Context

<!-- Please put here, in bullet points, links to the relevant docs page. A few starting template points are available
to get you started. -->

- [Link to documentation page](https://pyjanitor-devs.github.io/pyjanitor/api/functions/#janitor.functions.groupby_agg)
- [Link to exact file to be edited](https://github.com/pyjanitor-devs/pyjanitor/blob/dev/janitor/functions/groupby_agg.py)



Discussion:
@thatlittleboy thanks for the observation! kindly submit a PR for this fix. thanks!

Resolving patch:
diff --git a/janitor/functions/groupby_agg.py b/janitor/functions/groupby_agg.py
--- a/janitor/functions/groupby_agg.py
+++ b/janitor/functions/groupby_agg.py
@@ -35,7 +35,7 @@ def groupby_agg(
                                            agg='mean',
                                            agg_column_name="col1"
                                            new_column_name='col1_mean_by_group',
-                                           dropna = True/False)
+                                           dropna=True/False)
     ```
 
     Functional usage syntax:
@@ -56,6 +56,41 @@ def groupby_agg(
 
     Method chaining usage syntax:
 
+    ```python
+    df.groupby_agg(
+        by=['group', 'var1'],
+        agg='size',
+        agg_column_name='var1',
+        new_column_name='count',
+    )
+    ```
+
+           group  var1  count
+        0      1     1      4
+        1      1     1      4
+        2      1     1      4
+        3      1     1      4
+        4      1     2      1
+        5      2     1      1
+        6      2     2      3
+        7      2     2      3
+        8      2     2      3
+        9      2     3      1
+
+    If the data has null values,
+    you can include the null values by passing `False` to `dropna`;
+    this feature was introduced in Pandas 1.1:
+
+            name   type  num  nulls
+        0  black  chair    4    1.0
+        1  black  chair    5    1.0
+        2  black   sofa   12    NaN
+        3    red   sofa    4    NaN
+        4    red  plate    3    3.0
+
+    Let's get the count, including the null values,
+    grouping on `nulls` column:
+
     ```python
 
         df.groupby_agg(


Test patch: