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Extended line over 2 control points

Asked 2012-02-05T11:40:02.493
21

In matplotlib we can draw lines using at least 2 methods:

  1. plt.plot

    plt.plot([1,2],[1,2],color='k',marker='o')

  2. Line2D method

    line = lines.Line2D([0.3,0.6],[0.9,0.3],linestyle='dashed',color='k')
plt.axes().add_line(line)

I suspect both methods are the same in implementation of course. But anyway, it draws a line exactly between 2 stated points. Sometimes I need to extend line over those 2 points up to graph limits. Sure I can calculate it in form of y=ax+b, but does anybody know way easier?

Perfect case if I can just put some additional option, but I wasn't able to find it.

python matplotlib
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2 Answers

8

After good lunch I was able to find a way using numpy.

def drawLine2P(x,y,xlims):
    xrange = np.arange(xlims[0],xlims[1],0.1)
    A = np.vstack([x, np.ones(len(x))]).T
    k, b = np.linalg.lstsq(A, y)[0]
    plt.plot(xrange, k*xrange + b, 'k')
answered 2012-02-05T21:55:21.080
3

Little late on this, but I just came across this while googling. I was also sick of not being able to do this in matplotlib, so I wrote abline_plot. It includes callbacks to update a 2D line if the axes limits are changed.

Search for the abline_plot examples in the link below.

http://statsmodels.sourceforge.net/devel/examples/generated/example_interactions.html

Documentation:

http://statsmodels.sourceforge.net/devel/generated/statsmodels.graphics.regressionplots.abline_plot.html#statsmodels.graphics.regressionplots.abline_plot

Implementation:

https://github.com/statsmodels/statsmodels/blob/master/statsmodels/graphics/regressionplots.py#L572

Edit: A simpler one that doesn't update

import matplotlib.pyplot as plt
from matplotlib import lines as mpl_lines

def slope_from_points(point1, point2):
    return (point2[1] - point1[1])/(point2[0] - point1[0])

def plot_secant(point1, point2, ax):
    # plot the secant
    slope = slope_from_points(point1, point2)
    intercept = point1[1] - slope*point1[0] 
    # update the points to be on the axes limits
    x = ax.get_xlim()
    y = ax.get_ylim()
    data_y = [x[0]*slope+intercept, x[1]*slope+intercept]
    line = mpl_lines.Line2D(x, data_y, color='red')
    ax.add_line(line)
    return ax.figure()
answered 2013-01-26T21:59:03.917

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