Lasso Selector Demo Sgskip

Introduction

In this lab, you will learn how to interactively select data points with the lasso tool using Python Matplotlib. You will be plotting a scatter plot and then selecting a few points by drawing a lasso loop around the points on the graph. To draw, just click on the graph, hold, and drag it around the points you need to select.

VM Tips

After the VM startup is done, click the top left corner to switch to the Notebook tab to access Jupyter Notebook for practice.

Sometimes, you may need to wait a few seconds for Jupyter Notebook to finish loading. The validation of operations cannot be automated because of limitations in Jupyter Notebook.

If you face issues during learning, feel free to ask Labby. Provide feedback after the session, and we will promptly resolve the problem for you.

Import Libraries

Import the necessary libraries, including numpy, Path and LassoSelector.

import numpy as np
from matplotlib.path import Path
from matplotlib.widgets import LassoSelector

Create the Selector Class

Create the SelectFromCollection class that will select indices from a Matplotlib collection using LassoSelector.

class SelectFromCollection:
    """
    Select indices from a matplotlib collection using `LassoSelector`.

    Selected indices are saved in the `ind` attribute. This tool fades out the
    points that are not part of the selection (i.e., reduces their alpha
    values). If your collection has alpha < 1, this tool will permanently
    alter the alpha values.

    Note that this tool selects collection objects based on their *origins*
    (i.e., `offsets`).

    Parameters
    ----------
    ax : `~matplotlib.axes.Axes`
        Axes to interact with.
    collection : `matplotlib.collections.Collection` subclass
        Collection you want to select from.
    alpha_other : 0 <= float <= 1
        To highlight a selection, this tool sets all selected points to an
        alpha value of 1 and non-selected points to *alpha_other*.
    """

    def __init__(self, ax, collection, alpha_other=0.3):
        self.canvas = ax.figure.canvas
        self.collection = collection
        self.alpha_other = alpha_other

        self.xys = collection.get_offsets()
        self.Npts = len(self.xys)

        ## Ensure that we have separate colors for each object
        self.fc = collection.get_facecolors()
        if len(self.fc) == 0:
            raise ValueError('Collection must have a facecolor')
        elif len(self.fc) == 1:
            self.fc = np.tile(self.fc, (self.Npts, 1))

        self.lasso = LassoSelector(ax, onselect=self.onselect)
        self.ind = []

    def onselect(self, verts):
        path = Path(verts)
        self.ind = np.nonzero(path.contains_points(self.xys))[0]
        self.fc[:, -1] = self.alpha_other
        self.fc[self.ind, -1] = 1
        self.collection.set_facecolors(self.fc)
        self.canvas.draw_idle()

    def disconnect(self):
        self.lasso.disconnect_events()
        self.fc[:, -1] = 1
        self.collection.set_facecolors(self.fc)
        self.canvas.draw_idle()

Create the Scatter Plot

Create a scatter plot using randomly generated data.

np.random.seed(19680801)
data = np.random.rand(100, 2)

subplot_kw = dict(xlim=(0, 1), ylim=(0, 1), autoscale_on=False)
fig, ax = plt.subplots(subplot_kw=subplot_kw)
pts = ax.scatter(data[:, 0], data[:, 1], s=80)
selector = SelectFromCollection(ax, pts)

Accept the Selected Points

Accept the selected points using the enter key and print them to the console.

def accept(event):
    if event.key == "enter":
        print("Selected points:")
        print(selector.xys[selector.ind])
        selector.disconnect()
        ax.set_title("")
        fig.canvas.draw()

fig.canvas.mpl_connect("key_press_event", accept)
ax.set_title("Press enter to accept selected points.")

plt.show()

Summary

In this lab, you learned how to interactively select data points with the lasso tool using Python Matplotlib. You created a scatter plot, selected a few points by drawing a lasso loop around the points on the graph, and then accepted the selected points using the enter key.