When working with data visualization in Python, Altair is a powerful library that allows you to create stunning and interactive visualizations. In this article, we will explore how to create a horizontal bar graph using Altair, with two variables displayed in the same frame.

Option 1: Using the `alt.Chart` method

The first option to create a horizontal bar graph with two variables in the same frame is by using the `alt.Chart` method. This method allows us to create a chart object and specify the data source, as well as the encoding for the x and y axes.

import altair as alt
import pandas as pd

# Create a sample dataframe
data = pd.DataFrame({
    'Category': ['A', 'B', 'C', 'D'],
    'Variable1': [10, 20, 30, 40],
    'Variable2': [15, 25, 35, 45]
})

# Create the chart object
chart = alt.Chart(data).mark_bar().encode(
    x='Variable1',
    y='Category',
    color=alt.value('blue')
)

# Add the second variable as a layer
chart += alt.Chart(data).mark_bar().encode(
    x='Variable2',
    y='Category',
    color=alt.value('red')
)

# Display the chart
chart.show()

In this code snippet, we first import the necessary libraries, including Altair and Pandas. Then, we create a sample dataframe with two variables: `Variable1` and `Variable2`. Next, we create the chart object using the `alt.Chart` method and specify the data source as the dataframe.

We then use the `encode` method to specify the encoding for the x and y axes, as well as the color of the bars. We set the x-axis to `Variable1`, the y-axis to `Category`, and the color to blue. This creates the first layer of the bar graph.

To add the second variable as a layer, we create another chart object using the `alt.Chart` method and specify the same data source. We then use the `encode` method to set the x-axis to `Variable2`, the y-axis to `Category`, and the color to red. This creates the second layer of the bar graph.

Finally, we display the chart using the `show` method.

Option 2: Using the `alt.hconcat` method

The second option to create a horizontal bar graph with two variables in the same frame is by using the `alt.hconcat` method. This method allows us to concatenate multiple charts horizontally.

import altair as alt
import pandas as pd

# Create a sample dataframe
data = pd.DataFrame({
    'Category': ['A', 'B', 'C', 'D'],
    'Variable1': [10, 20, 30, 40],
    'Variable2': [15, 25, 35, 45]
})

# Create the first chart
chart1 = alt.Chart(data).mark_bar().encode(
    x='Variable1',
    y='Category',
    color=alt.value('blue')
)

# Create the second chart
chart2 = alt.Chart(data).mark_bar().encode(
    x='Variable2',
    y='Category',
    color=alt.value('red')
)

# Concatenate the charts horizontally
chart = alt.hconcat(chart1, chart2)

# Display the chart
chart.show()

In this code snippet, we follow a similar approach as in the first option. However, instead of adding the second variable as a layer, we create two separate chart objects: `chart1` and `chart2`. Each chart object represents a bar graph for a specific variable.

We then use the `alt.hconcat` method to concatenate the two charts horizontally, creating a single chart object that displays both variables in the same frame.

Finally, we display the chart using the `show` method.

Option 3: Using the `alt.layer` method

The third option to create a horizontal bar graph with two variables in the same frame is by using the `alt.layer` method. This method allows us to layer multiple charts on top of each other.

import altair as alt
import pandas as pd

# Create a sample dataframe
data = pd.DataFrame({
    'Category': ['A', 'B', 'C', 'D'],
    'Variable1': [10, 20, 30, 40],
    'Variable2': [15, 25, 35, 45]
})

# Create the first chart
chart1 = alt.Chart(data).mark_bar().encode(
    x='Variable1',
    y='Category',
    color=alt.value('blue')
)

# Create the second chart
chart2 = alt.Chart(data).mark_bar().encode(
    x='Variable2',
    y='Category',
    color=alt.value('red')
)

# Layer the charts on top of each other
chart = alt.layer(chart1, chart2)

# Display the chart
chart.show()

In this code snippet, we create two separate chart objects: `chart1` and `chart2`, similar to the second option. We then use the `alt.layer` method to layer the two charts on top of each other, creating a single chart object that displays both variables in the same frame.

Finally, we display the chart using the `show` method.

After exploring these three options, it is clear that the best option depends on the specific requirements of your project. If you prefer to add the second variable as a layer, Option 1 is a suitable choice. If you prefer to concatenate the charts horizontally, Option 2 is a good option. Lastly, if you prefer to layer the charts on top of each other, Option 3 is a viable choice. Consider the layout and visual representation you want to achieve to determine the best option for your needs.