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POellipses with various angles and eccentricities in matplotlib
 

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  1. POellipses with various angles and eccentricities in matplotlib
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    <p>I want to make a plot with ellipses as markers. Here is a sample code making one large ellipse that for now is actually a circle. </p> <pre><code>#! /usr/bin/env python3.2 import numpy as np import pylab import matplotlib.pyplot as plt from matplotlib.backends.backend_pdf import PdfPages from matplotlib.patches import Ellipse PlotFileName="test.pdf" pdf = PdfPages(PlotFileName) fig=plt.figure(1) ax1=fig.add_subplot(111) x_lim=3 plt.xlim([0,x_lim]) plt.ylim([0,x_lim]) F=pylab.gcf() DefSize = F.get_size_inches() #These lines have a true angle of 45 degrees, only as a reference: offset_along_x=x_lim-(x_lim/ax_ratio) ax1.plot([offset_along_x/2, x_lim-(offset_along_x/2)], [0, x_lim], "b") ax1.plot([offset_along_x/2, x_lim-(offset_along_x/2)], [x_lim, 0], "b") e=0.0 theta=0 maj_ax=2 min_ax=maj_ax*np.sqrt(1-e**2) xconst=(DefSize[1]/DefSize[0])*np.cos(theta*np.pi/180)-np.sin(theta*np.pi/180) yconst=np.cos(theta*np.pi/180)+(DefSize[1]/DefSize[0])*np.sin(theta*np.pi/180) print("xconstant= {}".format(xconst)) print("yconstant= {}".format(yconst)) ax1.add_artist(Ellipse((x_lim/2, x_lim/2), xconst*maj_ax, yconst*min_ax, angle=theta, facecolor="green", edgecolor="black",zorder=2, alpha=0.5)) pdf.savefig(fig) pdf.close() plt.close() </code></pre> <p>Although in this simplified case, <code>ax1.axis("equal")</code> will give a pure circle, but in my final plot, this command will ruin the whole plot (the scales aren't equal). So I want to make a general purpose ellipse tool without using <code>ax1.axis("equal")</code>. As you see, you can set the eccentricity and the inclination angle of the major axis in this program. </p> <p><strong>The problem:</strong> The problem seems to be that I don't understand how matplotlib rotates its images. If you change the value of <code>theta</code> here to a value other than 0 or 90, the object will no longer be a circle. with <code>ax1.axis("equal")</code> the output is a circle now matter what the value of <code>theta</code> is. So my first problem is this: what should I do to keep the output as a circle while changing <code>theta</code>. I assume that once I fix this, it will also work for an ellipse. Could someone please help me with this? I would really appreciate it.</p>
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    1. USmakhlaghi
    1. USmakhlaghi
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    1. COI'v noticed that there exists [Circle](http://matplotlib.sourceforge.net/api/artist_api.html#matplotlib.patches.Circle) in the patches, why don't you just use it instead of Ellipse?
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      1. POellipses with various angles and eccentricities in matplotlib
      1. USMayli
    2. COI want to make an ellipse finally. I am making a circle now to calibrate my program. If it works correctly with a circle, it will work with an ellipse.
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      1. POellipses with various angles and eccentricities in matplotlib
      1. USmakhlaghi
    3. COI think you're just confused on data coordinates vs display coordinates. Currently the ellipse is being plotted in _data_ coordinates. You seem to want the rotation, width, and height in _display_ coordinates, but the center in data coordinates, correct? If so, you just need to define the correct transform. I'm traveling right now, so I can't give an example, but this tutorial is a good start: http://matplotlib.sourceforge.net/users/transforms_tutorial.html You'll want a blended transform.
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      1. POellipses with various angles and eccentricities in matplotlib
      1. USJoe Kington
 

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