How to change font in $$ which is in math form in python

Question

When I tried to polt a figure with matplotlib, I wrote the x axis label with both text and "math text". Because I have to write a chemical formula in the label, it was written as '$CO_2$ concentration'. The question is that I hope the font should be times new roman, but I cannot change the font in the dollar sign somehow. Is there anyone who can help me fix it? Thank you very much!

import numpy as np 
import matplotlib.pyplot as plt
import pandas as pd

xf1 = pd.read_excel('1812_GPT.xlsx',sheetname= 'PVD_CO2CH4_600',usecols=[1])
deltapx1 = pd.read_excel('1812_GPT.xlsx',sheetname= 'PVD_CO2CH4_600',usecols=[3])
Px1 = pd.read_excel('1812_GPT.xlsx',sheetname= 'PVD_CO2CH4_600',usecols=[5])

ax1 = plt.subplot(111) 

l1, = ax1.plot(xf1,Px1,'s',markerfacecolor='black')

font1 = {'family' : 'Times New Roman',
'weight' : 'normal',
'size'   : 14,
 }

ax1.set_xlabel(r'$CO_2$ pressure', font1)
ax1.set_ylabel(r'$CO_2$ concentration', font1)

plt.show()

This is the picture link, you may see the pic and find that "CO2" is not in Times new roman. https://flic.kr/p/2dmh8pj

Answer 1

I don't think it's easily possible to change any mathtext to an arbitrary font. However, in case of "CO_2" that consists only of regular symbols, you may use \mathdefault{} to make the mathtext use the symbols from the regular font.

import matplotlib.pyplot as plt

plt.rcParams["font.family"] = "serif"
plt.rcParams["font.serif"] = ["Times New Roman"] + plt.rcParams["font.serif"]

fig, ax = plt.subplots()

ax.set_xlabel(r'$\mathdefault{CO_2}$ pressure')
ax.set_ylabel(r'$\mathdefault{CO_2}$ concentration')

plt.show()

Something like r"$\mathdefault{\sum_\alpha^\beta\frac{i}{9}}$ would still be rendered in the usual default math fontset (except the "i" and the 9, which are of course present in Times New Roman).

For the general case, you may also change the complete math fontset to any of the available, cm, stix, stixsans, dejavuserif, dejavusans. The closest to "Times New Roman" would be stix.

import matplotlib.pyplot as plt

rc = {"font.family" : "serif", 
      "mathtext.fontset" : "stix"}
plt.rcParams.update(rc)
plt.rcParams["font.serif"] = ["Times New Roman"] + plt.rcParams["font.serif"]


fig, ax = plt.subplots()

ax.set_xlabel(r'$CO_2$ pressure')
ax.set_ylabel(r'$CO_2$ concentration')

plt.show()

A general recommendation for reading would be the MathText tutorial.

Answer 2

I find it better to define the whole font family at once, rather than independently (assuming you want the same font). Give this a try

plt.rc('text', usetex=True )
plt.rc('font', family='Times New Roman', weight='normal', size=14)
plt.rcParams['mathtext.fontset'] = 'Times New Roman'
ax1.set_xlabel('$CO_2$ pressure')
ax1.set_ylabel('$CO_2$ concentration')