Plotting a circle-matplotlib

Question

I am trying to plot a circle on a grid. The code that I have written is as follows:

import pyplot as plt
from pyplot import Figure, subplot

fig=plt.figure(1)
plt.axis([0,400,0,400])
ax=fig.add_subplot(1,1,1)
circ=plt.Circle((200,200), radius=10, color='g', fill=False)
ax.add_patch(circ)
plt.show()

Now, I want the center of the circle to be the center of the graph, that is, (200,200) in this example. In case of other cases I want it to automatically choose the centre depending on the size that us set. Can this be in some way?

To make it clearer I want to get the x-axis and the y-axis range so as to find the mid point of the grid. How do I proceed?

score 4 · Answer 1 · answered Mar 31 '14 at 10:13

Your x-axis and y-axis ranges are in your code right here:

plt.axis([0,400,0,400])

So all you would need is leverage on this a bit like so:

x_min = 0
x_max = 400
y_min = 0
y_max = 400

circle_x = (x_max-x_min)/2.
circle_y = (y_max-y_min)/2.

circ=plt.Circle((circle_x,circle_y), radius=10, color='g', fill=False)

If you want to catch x_min etc. from the command prompt then read out sys.argv.

score 0 · Answer 2 · edited May 23 '17 at 11:53

What you want may be ax.transAxes, here's the tutorial for coordinates transformation.

ax.transAxes means the coordinate system of the Axes; (0,0) is bottom left of the axes, and (1,1) is top right of the axes.

fig=plt.figure(1)
plt.axis([0,400,0,400])
ax=fig.add_subplot(1,1,1)
circ=plt.Circle((0.5,0.5), radius=0.2, color='g', fill=False,transform=ax.transAxes)
ax.add_patch(circ)
plt.show()

Note that the radius is also transformed into Axes coordinate. If you specify a radius larger than sqrt(2)/2 (about 0.7) you will see nothing in the plot.

If you want to plot a set of circles, it would be much simpler if you use the function circles here. For this problem,

fig=plt.figure(1)
plt.axis([0,400,0,400])
ax=fig.add_subplot(1,1,1)
circles(0.5, 0.5, 0.2, c='g', ax=ax, facecolor='none', transform=ax.transAxes)
plt.show()

A bit more, if you want see a real circle (instead of an ellipse) in your figure, you should use

ax=fig.add_subplot(1,1,1, aspect='equal')

or something like that.

Plotting a circle-matplotlib

2 Answers2