Welcome to our lesson on Heat Maps for Monthly Data Analysis. In this session, we'll dive deeper into the more complex visualizations using the Seaborn library to plot heat maps.

Heat maps are a superb tool for displaying multivariate datasets in a two-dimensional image. They visually represent data through colors, where different color gradients represent different values. This is very useful in fields like Data Science, as heat maps are powerful tools for exploring and understanding patterns in a given dataset.

In our context of analyzing air travel data, what if we could find out how the monthly passenger count has fluctuated over the years? Which month or year had the highest passenger count? Does the count exhibit a pattern or trend? Heat maps are a great tool to answer these questions, and we'll learn to do exactly that in this lesson.

Heat maps are generated using the Seaborn library, which builds on Matplotlib and integrates seamlessly with pandas data structures. Let's start by developing a heat map for monthly passenger trends in air travel.

We start by loading up the flights dataset, as before:

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1import seaborn as sns
2
3# Load the dataset
4flights = sns.load_dataset('flights')

Since our interest is on a year-by-year and month-by-month basis, a pivot table fits our requirements best. The pivot table will have months as rows, years as columns, and passenger counts as the cell values. Python's pandas library makes creating this pivot table straightforward:

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1# Pivot the dataset
2flights_pivot = flights.pivot(index="month", columns="year", values="passengers")
3print(flights_pivot)
4"""
5year   1949  1950  1951  1952  1953  1954  1955  1956  1957  1958  1959  1960
6month                                                                        
7Jan     112   115   145   171   196   204   242   284   315   340   360   417
8Feb     118   126   150   180   196   188   233   277   301   318   342   391
9Mar     132   141   178   193   236   235   267   317   356   362   406   419
10Apr     129   135   163   181   235   227   269   313   348   348   396   461
11May     121   125   172   183   229   234   270   318   355   363   420   472
12Jun     135   149   178   218   243   264   315   374   422   435   472   535
13Jul     148   170   199   230   264   302   364   413   465   491   548   622
14Aug     148   170   199   242   272   293   347   405   467   505   559   606
15Sep     136   158   184   209   237   259   312   355   404   404   463   508
16Oct     119   133   162   191   211   229   274   306   347   359   407   461
17Nov     104   114   146   172   180   203   237   271   305   310   362   390
18Dec     118   140   166   194   201   229   278   306   336   337   405   432
19"""

Now that we have our pivot table, we can create a heat map. We use Seaborn's heatmap() function, passing in our pivot table as an argument:

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1# Plot a heatmap
2sns.heatmap(flights_pivot)
3
4plt.show()

This heat map immediately provides insight into the passenger count over the years. The color gradient (warmer for higher values, cooler for lower values) makes it easy to spot patterns and trends over time.

Seaborn offers several parameters to customize heatmaps for better readability and presentation. Here, we'll revamp our heatmap by tinkering with these parameters:

• cmap: This parameter controls the colormap for the heat map. Different colormaps can be used to enhance the heat map's visual appeal and help interpret the data better.
• annot: If set to True, this parameter allows the data values to be written on each cell in the heat map.
• fmt: This is a string formatting code to use when adding annotations. While it's unnecessary when the annot is not True, you need to specify a string formatting code if you add annotations.
• linewidths: This parameter allows adding lines between each cell in the heatmap. This helps to distinguish between each cell, especially when the colors amongst cells do not vary greatly.
• cbar: This parameter adds a color bar to the heatmap when set to True. The color bar helps in understanding the color coding of the heatmap cells.
• center: This parameter defines the value at which to center the colormap. This is useful in cases where the heatmap cells take values diverging around zero.

Let's experiment with these parameters and enhance our heatmap:

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1# Detailed heat map
2sns.heatmap(flights_pivot, 
3            cmap='YlGnBu', # choosing a yellow-green-blue colormap
4            annot=True, # Turning on annotations
5            fmt="d", # displaying annotations as integer
6            linewidths=.5, # Add gridlines with width 0.5
7            cbar=True, # Include color bar
8            center = flights_pivot.loc["Jan", 1955] # Center colormap at the value of passengers in January 1955
9)
10plt.show()

Running this code block would produce a more detailed heat map. The numbers in each cell correspond to the actual count of passengers. There are lines of width 0.5 distinguishing every two heatmap cells. The color bar to the right of the heatmap serves as a reference for interpreting the heatmap colors. The center parameter ensures that the colormap's neutral point corresponds to the number of passengers in January 1955.

We can quickly spot a significant insight: as the years go on, the passenger count increases, indicating the growth in the air travel industry. Moreover, we can observe a pattern in passenger counts attaining a peak during the summers each year, displaying the seasonality in air travel.

In this case, "seasonality" refers to a periodic and consistent fluctuation in the number of passengers over different months of the year. This type of pattern often occurs when the observations are collected over time. For instance, a school might display seasonality in electricity consumption, using more during the academic year and less during the holidays.

Appreciation for your accomplishment! Now you know how to create and customize heat maps using Python and Seaborn, and how to extract valuable insights from them. Heat maps are a powerful tool for your data visualization skill set.

Remember the colors, patterns, and trends we identified in our heatmap? They answered our initial questions and painted a comprehensive picture of the growth and seasonality of air travel.

Next, it's time to reinforce and practice these skills! In the upcoming hands-on exercises, you will find basic exercises for beginners and challenges to test your new skills. Remember, the true art of learning to code is coding! Enjoy the thrill of these practice sessions, and best of luck!