Styjun

Using matplotlib for Python, how do I write put 'sqrt(2)' with the square root symbol instead of its decimal value in a plot legend? See the plot and code below. Please let me know if I need to elaborate. Thank you!

import matplotlib.pyplot as plt
import numpy as np


E = np.linspace(50000,200000,1000) # eV

a = 3.615*10**(-10) #m
h = 6.582*10**(-16) #eV s
m = 9.109*10**(-31) #kg
J = 6.242*10**(-18) #1eV to J

def T(x,E):
 return np.arcsin(x*np.sqrt(J*((np.pi**2 *h**2)/(a**2 * 4*m *E))))*1000

fig, ax = plt.subplots()
x = np.asarray([2*np.sqrt(2), 2, np.sqrt(3)])
for i in x:
 ax.plot(E,T(i,E),label='hkl=%s' % i)
 plt.ylabel('$Theta$ (mrad)')
 plt.xlabel('Energy (eV)')
 plt.legend(bbox_to_anchor=(1.01, 1), loc=1, borderaxespad=0.)

The plot this spits out is:

EDIT:
I should mention that I want this code to take any value in x and plot it, using an actual square root symbol in the legend instead of its decimal value.

You can use Unicode to write the square root symbol as a string and then add whatever number you are interested in. To write a square root symbol as a string, use 'u221A'.

One way to modify your code would be to turn x into a dictionary. So x='2u221A2': 2*np.sqrt(2), '2':2, 'u221A3':np.sqrt(3). Your for loop would become for k,v in x.items() where k and v are short for key and value and the first line in the loop would be ax.plot(E,T(v,E),label='hkl=%s' % k)

In terms of parsing a number and writing it out as a square root, that's a little more involved and would look something like this question on code review: https://codereview.stackexchange.com/questions/144041/reduce-square-root-to-simplest-radical-form

You can do the printing of the square root symbol pretty easily using LaTex in maplotlib, i.e.

s = 2
plt.plot(x, y, label=r'$sqrt$'.format(s))

The tricky part of course is using the right coefficients and roots, this is easier if you take a slightly different approach to generating x, something like this

import matplotlib.pyplot as plt
import numpy as np

E = np.linspace(50000,200000,1000) # eV

a = 3.615*10**(-10) #m
h = 6.582*10**(-16) #eV s
m = 9.109*10**(-31) #kg
J = 6.242*10**(-18) #1eV to J

def T(x,E):
 return np.arcsin(x*np.sqrt(J*((np.pi**2 *h**2)/(a**2 * 4*m *E))))*1000

fig, ax = plt.subplots()
k = [(2, 2), (2, 1), (1, 3)]
x = list()
labels = list()
for i in range(len(k)):
 x.append(k[i][0] * np.sqrt(k[i][1]))
 if k[i][0] == 1 and k[i][1] != 1:
 labels.append(r'hkl=$sqrt$'.format(k[i][1]))
 elif k[i][0] != 1 and k[i][1] == 1:
 labels.append(r'hkl=$$'.format(k[i][0]))
 elif k[i][0] != 1 and k[i][1] != 1:
 labels.append(r'hkl=$sqrt$'.format(k[i][0], k[i][1]))
 else:
 labels.append('hkl=1')

for i in range(len(x)):
 ax.plot(E,T(x[i],E),label=labels[i])
 plt.ylabel('$Theta$ (mrad)')
 plt.xlabel('Energy (eV)')
 plt.legend(bbox_to_anchor=(0.99, 0.98), loc=1, borderaxespad=0.)

This will take any values to construct x and print the square root symbols approriately (though I am sure there are better ways to do so)