Geopandas categories in legends¶

Introduction¶

I recently ran across a little wrinkle in plotting maps with Geopandas. In this particular case I was plotting a subset of a large DataFrame, and the Legends produced were misleading. This brief blog post show a workaround.

Implementation¶

Define environment¶

Import of packages used¶

In [1]:

import geopandas as gpd
import numpy as np
import pandas as pd

Notebook utilities used¶

In [2]:

%load_ext watermark

In [3]:

%load_ext lab_black

Get example dataset¶

We will use a sample built-in dataset that Geopandas provides to illustrate the glitch: first, plot and print the contents.

In [4]:

gpd.datasets.available

Out[4]:

['naturalearth_cities', 'naturalearth_lowres', 'nybb']

In [5]:

df = gpd.read_file(gpd.datasets.get_path("nybb"))
df.plot()

Out[5]:

<AxesSubplot:>

In [6]:

df.head()

Out[6]:

	BoroCode	BoroName	Shape_Leng	Shape_Area	geometry
0	5	Staten Island	330470.010332	1.623820e+09	MULTIPOLYGON (((970217.022 145643.332, 970227....
1	4	Queens	896344.047763	3.045213e+09	MULTIPOLYGON (((1029606.077 156073.814, 102957...
2	3	Brooklyn	741080.523166	1.937479e+09	MULTIPOLYGON (((1021176.479 151374.797, 102100...
3	1	Manhattan	359299.096471	6.364715e+08	MULTIPOLYGON (((981219.056 188655.316, 980940....
4	2	Bronx	464392.991824	1.186925e+09	MULTIPOLYGON (((1012821.806 229228.265, 101278...

Set Borough Name column as a categorical column¶

We would typically do this on a large DataFrame in order to improve efficiency

In [7]:

df['BoroName'] = df['BoroName'].astype('category')

List out the categories.

In [8]:

df['BoroName'].cat.categories

Out[8]:

Index(['Bronx', 'Brooklyn', 'Manhattan', 'Queens', 'Staten Island'], dtype='object')

Now request a plot, where the Boroughs are distinguished by color

In [9]:

df.plot(
    column='BoroName',
    legend=True,
    legend_kwds={'loc': 'upper left',},
)

Out[9]:

<AxesSubplot:>

Now we will plot just a subset of the data. We subset by Borough Code. First, we list all codes.

In [10]:

[code for code in df['BoroCode']]

Out[10]:

[5, 4, 3, 1, 2]

Now we mask out all those Boroughs that have a code number value not in 3, 4, or 5.

In [11]:

selected_codes = [3, 4, 5]
df2 = df[
    [code in selected_codes for code in df['BoroCode']]
].copy()
df2.plot(
    column='BoroName',
    legend=True,
    legend_kwds={'loc': 'upper left',},
)

Out[11]:

<AxesSubplot:>

Now you can see the problem: although we only plotted 3 Boroughs, the Legend shows five colors!

We can see that the categories have been unchanged by our subsetting operation.

In [12]:

df2['BoroName'].cat.categories

Out[12]:

Index(['Bronx', 'Brooklyn', 'Manhattan', 'Queens', 'Staten Island'], dtype='object')

Solution¶

We define a helper function that strips out unused categories

In [13]:

def strip_unused_categories(
    df: pd.DataFrame, column_name: str
):
    '''
    strip_unused_categories:  remove unused categories from a categorical colm definition
    
    Parameters:
    df: pandas DataFrame, with one column being  categorical
    
    column_name: str - name of categorical column to be pruned
    
    Returns:
    None
    
    Side Effects:
    Categories of referenced columns might be changed
    
    Example:
    If we have: 
        0     Blue
        1    Green
        2     Blue
        3    Green
        4    Brown
        Name: color, dtype: category
        Categories (4, object): [Blue, Brown, Green, Red]
    
    we want:
        0     Blue
        1    Green
        2     Blue
        3    Green
        4    Brown
        Name: color, dtype: category
        Categories (3, object): [Blue, Green, Brown]
        
        Usage Example:
        df = pd.DataFrame(
            {
                'color': np.random.choice(
                    ['Blue', 'Green', 'Brown', 'Red'], 50
                )
            }
        )

        df.color = df.color.astype('category')
        print(df.color.cat.categories)

        df = df.query('color != "Brown"')
        strip_unused_categories(df, 'color')
        print(df.color.cat.categories)
        
        ->
        Index(['Blue', 'Brown', 'Green', 'Red'], dtype='object')
        Index(['Blue', 'Green', 'Red'], dtype='object')
        
    '''

    # get the all the current category names as a list
    current = list(df[column_name].cat.categories)

    # get list on unique categories actually used
    used = df[column_name].unique()

    for cat_name in current:
        if cat_name not in used:
            df[column_name] = df[
                column_name
            ].cat.remove_categories(cat_name)
        # end if
    # end for


# end strip_unused_categories

Using the helper function, we prune the category list

In [14]:

strip_unused_categories(df2, 'BoroName')

Now the plot has a legend that makes sense!

In [15]:

df2.plot(
    column='BoroName',
    legend=True,
    legend_kwds={'loc': 'upper left',},
)

Out[15]:

<AxesSubplot:>

Just checking our prune operation worked as expected.

In [16]:

df2['BoroName'].cat.categories

Out[16]:

Index(['Brooklyn', 'Queens', 'Staten Island'], dtype='object')

Just as a side note, the misleading Legend also appears if other means of subsetting are used.

In [17]:

df.query('BoroCode>2').plot(
    column='BoroName',
    legend=True,
    legend_kwds={'loc': 'upper left',},
)

Out[17]:

<AxesSubplot:>

In [ ]:

Reproducability¶

In [18]:

%watermark -iv

pandas   : 1.5.2
numpy    : 1.23.5
geopandas: 0.9.0

In [19]:

%watermark

Last updated: 2024-02-04T19:12:33.113777+10:00

Python implementation: CPython
Python version       : 3.9.15
IPython version      : 8.8.0

Compiler    : MSC v.1916 64 bit (AMD64)
OS          : Windows
Release     : 10
Machine     : AMD64
Processor   : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
CPU cores   : 8
Architecture: 64bit

In [20]:

%watermark -co

conda environment: mapping2

In [ ]:

Geopandas and Categorical Legends