\text does not work in a matplotlib label

\text won’t work because it requires the amsmath package (not included in mathtext – the math rendering engine of matplotlib). So you basically have two options:

• use latex based font rendering

from matplotlib import pyplot as plt
import matplotlib as mpl
mpl.rcParams['text.usetex'] = True
mpl.rcParams['text.latex.preamble'] = [r'\usepackage{amsmath}'] #for \text command
plt.plot([1,2,3])
plt.title(r"$f_{\text{cor, r}}$")
plt.show()

2. use mathtext but use \mathrm instead of \text

from matplotlib import pyplot as plt
import matplotlib as mpl
mpl.rcParams['text.usetex'] = False  # not really needed
plt.plot([1,2,3])
plt.title(r"$f_{\mathrm{cor, r}}$")
plt.show()

The latter approach creates a figure like

Be aware that unlike with the \text command, spaces inside the \mathrm environment are not respected. If you want more space between the variables you have to use latex style commands (\<space>, \;, …).