In sympy plotting, how can I get a plot with a fixed aspect ratio?

I'm not sure if this is covered in Sympy's stable API, but you can extract matplotlib's figure and axis instance and the use standard matplotlib calls to change the appearance of your plot:

import matplotlib.pyplot as plt
import sympy as sy

x, y = sy.symbols('x y')
p1 = sy.plot_implicit(sy.Eq(x**2 +y**2, 4))
fg, ax = p1._backend.fig, p1._backend.ax  # get matplotib's figure and ax

# Use matplotlib to change appearance: 
ax.axis('tight')  # list of float or {‘on’, ‘off’, ‘equal’, ‘tight’, ‘scaled’, ‘normal’, ‘auto’, ‘image’, ‘square’}
ax.set_aspect("equal") # 'auto', 'equal' or a positive integer is allowed
ax.grid(True)
fg.canvas.draw()


plt.show()  # enter matplotlib's event loop (not needed in Jupyter)

This gives: