How to Create Hovering Annotations for Scatter Plots in Matplotlib?

How to Add Hovering Annotations to a Plot

Annotating points on a scatter plot is a common task when working with data. Matplotlib, a Python library for creating 2D plots, provides a simple method for adding fixed annotations to a plot using the annotate command. However, this approach can become impractical when dealing with numerous data points as the plot might become cluttered.

Fortunately, there is a solution that involves creating dynamic annotations that only appear when the cursor hovers over a specific data point. This method requires a slight modification of the annotate function in conjunction with a callback function to handle cursor events.

Here's an example code demonstrating the implementation:

import matplotlib.pyplot as plt
import numpy as np; np.random.seed(1)

x = np.random.rand(15)
y = np.random.rand(15)
names = np.array(list("ABCDEFGHIJKLMNO"))
c = np.random.randint(1, 5, size=15)

norm = plt.Normalize(1, 4)
cmap = plt.cm.RdYlGn

fig, ax = plt.subplots()
sc = plt.scatter(x, y, c=c, s=100, cmap=cmap, norm=norm)

annot = ax.annotate("", xy=(0, 0), xytext=(20, 20), textcoords="offset points",
                    bbox=dict(boxstyle="round", fc="w"),
                    arrowprops=dict(arrowstyle="->"))
annot.set_visible(False)

def update_annot(ind):
    pos = sc.get_offsets()[ind["ind"][0]]
    annot.xy = pos
    text = "{}, {}".format(" ".join(list(map(str, ind["ind"]))),
                           " ".join([names[n] for n in ind["ind"]]))
    annot.set_text(text)
    annot.get_bbox_patch().set_facecolor(cmap(norm(c[ind["ind"][0]])))
    annot.get_bbox_patch().set_alpha(0.4)

def hover(event):
    vis = annot.get_visible()
    if event.inaxes == ax:
        cont, ind = sc.contains(event)
        if cont:
            update_annot(ind)
            annot.set_visible(True)
            fig.canvas.draw_idle()
        else:
            if vis:
                annot.set_visible(False)
                fig.canvas.draw_idle()

fig.canvas.mpl_connect("motion_notify_event", hover)

plt.show()

This code adds a toolTip that appears when the mouse hovers over a data point, displaying its coordinates and name. The update_annot function dynamically updates the annotation's position and content depending on the hovered point.

This approach allows for a clutter-free visualization with easily accessible information about each data point, making it suitable for interactive data exploration.

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