Lines such as the following will work fine with matplotlib:
plt.xlabel('$\gamma$')
plt.xlabel('$\Gamma$')
plt.xlabel('$\pi$')
plt.xlabel('$\Pi$')
plt.xlabel('$\beta$')
plt.xlabel('$\Beta$')
plt.xlabel('$\alpha$')
plt.xlabel('$\Alpha$')
ValueError:
\Beta
^
Unknown symbol: \Beta (at char 0), (line:1, col:1)
Ed Smith's answer (see comment above) did it. I just had to use this line:
plt.xlabel(r'$\beta$')