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How to label lines in matplotlib

Understanding Labels in Matplotlib

Imagine you're creating a visual story with data, and your chart is the canvas. Just like characters in a story need names, lines in a chart need labels to tell them apart. Matplotlib, a popular plotting library in Python, allows you to add these informative labels to your lines, making your plots not only more informative but also easier to understand.

The Basics of Line Labeling

Let's start with the basics. In Matplotlib, a line can be labeled using the label parameter when you plot it. This label is what will be displayed in the legend, a small area on the plot that explains the symbols, colors, or line types used.

Here's a simple example:

import matplotlib.pyplot as plt

# Data points
x = [1, 2, 3, 4, 5]
y = [2, 3, 5, 7, 11]

# Plotting the line
plt.plot(x, y, label='Prime Numbers')

# Adding a legend to the plot

# Display the plot

In this example, we plot a line that represents prime numbers. The label='Prime Numbers' part is where we give our line a name. When we call plt.legend(), Matplotlib looks for any labels we've defined and creates a legend for us.

Customizing the Legend

Sometimes, the default legend isn't quite right for your needs. You might want to change its location, font size, or even the number of columns. Fortunately, Matplotlib gives you the tools to do just that.

For instance, if you want to move the legend to the upper left corner, you can do so like this:

plt.legend(loc='upper left')

The loc parameter controls the location of the legend. You can use simple strings like 'upper left', 'lower right', etc., or you can specify a location using a tuple of x and y values, which represent the lower-left point of the legend in the figure's coordinate space.

Labeling Multiple Lines

When your story has more than one character, you introduce them one by one. Similarly, when your plot has multiple lines, you label each one as you plot it.

# More data points
y2 = [1, 4, 9, 16, 25]

# Plotting two lines
plt.plot(x, y, label='Prime Numbers')
plt.plot(x, y2, label='Squares')

# Adding a legend

# Display the plot

Now our plot has two labeled lines, each represented in the legend. Just like before, plt.legend() finds these labels and includes them both.

Styling Your Labels

To make your labels stand out or fit in with your plot's aesthetic, you can style them. Matplotlib allows you to change the font size, color, and more.

Here's how you can change the font size of the labels in the legend:


The fontsize parameter accepts predefined strings like 'small', 'medium', 'large', or numerical values to specify the font size directly.

Interactive Labeling with Annotation

Sometimes a legend isn't enough. You might want to point out specific points on your line and give more context. This is where annotations come in.

Annotations are like little comments you can add to specific points on your plot. Here's how you can annotate a point:

# Highlighting the number 5 in our prime number series
plt.annotate('Five', xy=(3, 5), xytext=(4, 6),
             arrowprops=dict(facecolor='black', shrink=0.05))

The xy parameter is the point you want to annotate, and xytext is where you want the text to be placed. arrowprops lets you style the arrow that points from your text to the point.

Understanding Axes and Figures

To fully grasp labeling, you need to understand two key concepts in Matplotlib: axes and figures. Think of the figure as the entire picture frame and the axes as the canvas where you draw your lines. In most cases, when you're plotting with Matplotlib, you're working with axes.

Here's how you can create a figure and axes explicitly:

fig, ax = plt.subplots()

# Now we use ax to plot
ax.plot(x, y, label='Prime Numbers')
ax.plot(x, y2, label='Squares')

# We also use ax to create the legend

# Finally, we display the figure

Using ax instead of plt directly gives you more control over your plot.

Labeling Lines with a Loop

If you have many lines to label, writing them out one by one can be tedious. A loop can help automate this process. Let's say we have a list of data series and a corresponding list of labels:

data_series = [y, y2]
labels = ['Prime Numbers', 'Squares']

for data, label in zip(data_series, labels):
    plt.plot(x, data, label=label)


Here, zip combines our data series and labels so we can loop over them together, plotting each line with its label.


Labeling lines in Matplotlib is like naming characters in a story—it brings clarity and understanding to the plot. With the ability to customize legends, annotate specific points, and handle multiple lines efficiently, you can guide your audience through the narrative of your data with ease.

As you continue your journey in programming, remember that your code is not just about getting the job done; it's about telling a story that others can follow. So, always label your lines, and let your data speak clearly and effectively. Whether you're a data scientist, an engineer, or just a curious learner, the art of clear communication through plots is a valuable skill in your toolkit. Happy plotting, and may your data stories always captivate and inform!