4.7. Setting axes limits

When we create a plot interactively, we can zoom on a particular section using the figure’s Zoom and Pan buttons. It is also possible to zoom programmatically while we create the figure, using plt.axis.

4.7.1. Rectangular zoom

Let’s create some random data:

import matplotlib.pyplot as plt

random_data = [
    0.60921838, 0.80049539, 0.11593785, 0.90703271, 0.75252107,
    0.72976933, 0.93364321, 0.14401492, 0.35255034, 0.68628490,
    0.76262073, 0.48124243, 0.73965093, 0.21237858, 0.63081351,
    0.75098067, 0.87658458, 0.68637267, 0.30947229, 0.19968510,
    0.59710015, 0.98464582, 0.05031455, 0.56996651, 0.74360835,
    0.30706612, 0.92693383, 0.63387122, 0.30740088, 0.46847444,
    0.99559865, 0.84777408, 0.21486266, 0.65472302, 0.51600203,
    0.3724475 , 0.0579805 , 0.62423827, 0.33997655, 0.14256265,
    0.5013935 , 0.98862076, 0.58028518, 0.15716675, 0.83572021,
    0.0119542 , 0.7257411 , 0.99901993, 0.69608303, 0.46573617
]

plt.plot(random_data, "o-");

To zoom on a specific portion of the plot, we call plt.axis, which takes for argument a list containing [xmin, xmax, ymin, ymax]. For example, to better see the portion between \(x \geq 20\) and \(x \leq 30\):

plt.plot(random_data, "o-")
plt.axis([20, 30, 0, 1]);

4.7.2. Square zoom

By defaults, plots are rectangular and will adapt their shape to optimize the scale on both x and y axes. However, in some circumstances, we need both x and y axes to have the same scale, e.g., to plot the (x, y) trajectory of a point in space. To set the same scale to both axes, we use plt.axis("square").

For example, plotting the (x, y) coordinates of a true circle looks like an ellipse by default:

x = [ 1.        ,  0.96592583,  0.8660254 ,  0.70710678,  0.5       ,
      0.25881905,  0.        , -0.25881905, -0.5       , -0.70710678,
     -0.8660254 , -0.96592583, -1.        , -0.96592583, -0.8660254 ,
     -0.70710678, -0.5       , -0.25881905, -0.        ,  0.25881905,
      0.5       ,  0.70710678,  0.8660254 ,  0.96592583,  1.        ]
     
y = [ 0.        ,  0.25881905,  0.5       ,  0.70710678,  0.8660254 ,
      0.96592583,  1.        ,  0.96592583,  0.8660254 ,  0.70710678,
      0.5       ,  0.25881905,  0.        , -0.25881905, -0.5       ,
     -0.70710678, -0.8660254 , -0.96592583, -1.        , -0.96592583,
     -0.8660254 , -0.70710678, -0.5       , -0.25881905, -0.        ]
     
plt.plot(x, y, "o-");

This is solved by setting the axis to “square”:

plt.plot(x, y, "o-")
plt.axis("square");