save in CSV and plotting in real time

[buran]

Here is untested code with some changes to make it better

import serial                                          
import numpy as np                                    
import adafruit_max31856
import matplotlib.pyplot as plt                        
from drawnow import drawnow
import datetime
import math
import csv
import os
from collections import deque
from decimal import Decimal

# instead of list, use collections.deque
# deque has optional maxlen argument that set max number of elements
# if append new element and len exceed maxlen elemnt from the other side is removed
temp1 = deque(maxlen=50) # make deque with length 50
temp2 = deque(maxlen=50) # make deque with length 50
plt.ion()

def make_fig():  
    plt.title('My Super Beautiful Live Streaming Sensor Data :-)')      
    plt.grid(True)#Turn the grid on
    plt.plot()
    plt.xlabel('Time')                                
    plt.ylabel('Temperature')                          
    plt.plot(temp1, 'ro-',label='Temp1')              
    plt.plot(temp2, 'b^-',label='Temp2')
    plt.legend(loc='upper right')                      
    plt.show()


if __name__ == '__main__':
    ## user-defined params
    serialPort = 'COM4'                                                  
    out_path = r'C:\Users\Alphinity\Desktop\python3\'              
    file_name = "signalSerial.csv"                    
    ser = serial.Serial('COM4', 115200)
    out_file = os.path.join(out_path, file_name)

    print (f"Writing the serial stream into file: {out_file}")
    print (f" [to see the stream: tail -f {out_file} ]")
    print (" [to exit: ctrl+c (the elegant way :) ]")
    while True:                                
        line = ser.readline().decode().strip()
        if line:
            line = line.split(',') # get list of 2 strings
            line = [Decimal(temp.strip()) for temp in line] # make line list of two Decimal objects
            t1, t2 = line # unpack line into two variables t1 and t2
            temp1.append(t1)                      
            temp2.append(t2)
            drawnow(make_fig)
            plt.pause(.000001)
            with open(out_fiile, 'a') as f:
                wrtr = csv.writer(f)
                wrtr.writerow(line)

            # as alternative to last 3 lines
            with open(out_fiile, 'a') as f:
                f.write(f'{t1},{t2}\n')

I didn't change your drawing of the plot and definitely program structure could be better