How to Plot Real-Time Serial Data in Python with PySerial & PyQtGraph

1. Install pyserial and basic usage

Install the serial communication library with pip install pyserial . Common methods include serial.Serial(0) to open the first port, ser.portstr to view the port name, ser.write() to send data, ser.read() or ser.readline() to receive data, and ser.close() to close the port. You can set parameters such as baudrate , bytesize , parity , stopbits , and timeout .

2. Basic serial code

<code>import serial
portx = "COM5"
bps = 9600
timeout = 1
ser = serial.Serial(portx, int(bps), timeout=timeout, parity=serial.PARITY_NONE, stopbits=1)
if ser.isOpen():
    print("open success")
    ser.write("hello".encode())
    while True:
        line = ser.readline()
        if line:
            print(line)
else:
    print("open failed")
ser.close()
</code>

3. Using pyqtgraph for real-time plotting

Install the visualization libraries with pip install pyqtgraph and pip install PyQt5 . Pyqtgraph leverages NumPy and Qt's GraphicsView for fast 2D/3D plotting, making it suitable for large‑data and interactive applications.

<code>import pyqtgraph as pg
import numpy as np
import array
app = pg.mkQApp()
win = pg.GraphicsWindow()
win.setWindowTitle('pyqtgraph real‑time plot')
win.resize(800, 500)
historyLength = 100
p = win.addPlot()
p.showGrid(x=True, y=True)
p.setRange(xRange=[0, historyLength], yRange=[-1.2, 1.2], padding=0)
p.setLabel('left', 'y / V')
p.setLabel('bottom', 'x / point')
curve = p.plot()
idx = 0

def plotData():
    global idx
    tmp = np.sin(np.pi / 50 * idx)
    if len(data) < historyLength:
        data.append(tmp)
    else:
        data[:-1] = data[1:]
        data[-1] = tmp
    curve.setData(data)
    idx += 1

timer = pg.QtCore.QTimer()
timer.timeout.connect(plotData)
timer.start(50)
app.exec_()
</code>

4. Real‑time plotting with multithreading

Run serial reading in a separate thread and update the plot from the main thread.

<code>import array, serial, threading, numpy as np, pyqtgraph as pg
i = 0

def Serial():
    while True:
        n = mSerial.inWaiting()
        if n:
            dat = int.from_bytes(mSerial.readline(1), byteorder='little')
            if i < historyLength:
                data[i] = dat
                i += 1
            else:
                data[:-1] = data[1:]
                data[-1] = dat

def plotData():
    curve.setData(data)

if __name__ == "__main__":
    app = pg.mkQApp()
    win = pg.GraphicsWindow()
    win.setWindowTitle('pyqtgraph multithreaded plot')
    win.resize(800, 500)
    historyLength = 200
    data = np.zeros(historyLength, dtype='d')
    p = win.addPlot()
    p.showGrid(x=True, y=True)
    p.setRange(xRange=[0, historyLength], yRange=[0, 255], padding=0)
    p.setLabel('left', 'y / V')
    p.setLabel('bottom', 'x / point')
    curve = p.plot()
    mSerial = serial.Serial('COM24', 19200)
    if mSerial.isOpen():
        print('open success')
        mSerial.write('hello'.encode())
    else:
        print('open failed')
    th1 = threading.Thread(target=Serial)
    th1.start()
    timer = pg.QtCore.QTimer()
    timer.timeout.connect(plotData)
    timer.start(50)
    app.exec_()
</code>

5. Communicating with a microcontroller for continuous monitoring

Use a queue to buffer incoming bytes, convert signed values, and update the plot at a fixed interval.

<code>import pyqtgraph as pg, array, serial, threading, numpy as np, time
from queue import Queue
q = Queue()

def Serial():
    while True:
        n = mSerial.inWaiting()
        if n:
            dat = int.from_bytes(mSerial.readline(1), byteorder='little')
            if dat >> 7:
                dat = 256 - dat
                dat = -dat
            q.put(dat)

def plotData():
    global i
    if i < historyLength:
        data[i] = q.get()
        i += 1
    else:
        data[:-1] = data[1:]
        data[-1] = q.get()
    curve.setData(data)

if __name__ == "__main__":
    app = pg.mkQApp()
    win = pg.GraphicsWindow()
    win.setWindowTitle('pyqtgraph microcontroller plot')
    win.resize(800, 500)
    historyLength = 100
    data = np.zeros(historyLength, dtype='d')
    p = win.addPlot()
    p.showGrid(x=True, y=True)
    p.setRange(xRange=[0, historyLength], yRange=[-50, 50], padding=0)
    curve = p.plot()
    mSerial = serial.Serial('COM25', 19200)
    if mSerial.isOpen():
        print('open success')
        mSerial.write('hello'.encode())
    else:
        print('open failed')
    th1 = threading.Thread(target=Serial)
    th1.start()
    timer = pg.QtCore.QTimer()
    timer.timeout.connect(plotData)
    timer.start(1)
    app.exec_()
</code>