Matplotlib - multiple plots from serial data

Question

I am reading data from 4 different sensors using serial communication and want to plot each sensor data in a separate plot. My code is:

import serial
import matplotlib.pyplot as plt
import numpy as np

connected = False

comPort = 'COM4'        

ser = serial.Serial(comPort, 115200)    # Sets up serial connection (make sure baud rate is correct - matches Arduino)

while not connected:
    serin = ser.read()
    connected = True

plt.ion()                               # Sets plot to animation mode

fig1 = plt.figure()
fig2 = plt.figure()
fig3 = plt.figure()
fig4 = plt.figure()

length = 20                             # Determines length of data taking session (in data points); length/10 = seconds

w = [0]*length                          # Create empty variable of length of test
x = [0]*length               
y = [0]*length
z = [0]*length

wline, = plt.plot(w)                    # Sets up future lines to be modified
xline, = plt.plot(x)                    
yline, = plt.plot(y)
zline, = plt.plot(z)

plt.ylim(0,64535)                       # Sets the y axis limits - 16 bits resolution

for i in range(length):                 # While you are taking data
    data = ser.readline()               # Reads until it gets a carriage return (/n).
    sep = data.split()                  # Splits string into a list at the tabs

    w.append(int(sep[0]))               # Add new values as int to current list
    x.append(int(sep[1]))   
    y.append(int(sep[2]))
    z.append(int(sep[3]))

    del w[0]
    del x[0]
    del y[0]
    del z[0]

    wline.set_xdata(np.arange(len(w)))  # Sets wdata to new list length  
    xline.set_xdata(np.arange(len(x)))  
    yline.set_xdata(np.arange(len(y)))  
    zline.set_xdata(np.arange(len(z)))  

    wline.set_ydata(w)                  # Sets ydata to new lists 
    xline.set_ydata(x)                 
    yline.set_ydata(y)
    zline.set_ydata(z)

    print i
    print sep

    ax1 = fig1.add_subplot(111)
    ax1.plot(wline.set_ydata(w))
#    ax1.plot(sep[0])
    plt.pause(0.001)                   
    plt.grid(True)
    plt.draw()                         # Draws new plot

    ax2 = fig2.add_subplot(111)
    ax2.plot(xline.set_ydata(x))
    plt.pause(0.001)                   
    plt.grid(True)
    plt.draw()                         # Draws new plot

    ax3 = fig3.add_subplot(111)
    ax3.plot(yline.set_ydata(y))
    plt.pause(0.001)                   
    plt.grid(True)
    plt.draw()                         # Draws new plot

    ax4 = fig4.add_subplot(111)
    ax4.plot(zline.set_ydata(z)) 
    plt.pause(0.001)                   
    plt.grid(True)
    plt.draw()                         # Draws new plot

plt.show()

ser.close()                             # Closes serial connection

The data is acquired correctly and the the 4 figures are generated, however only the last one is plotting the data. Furthermore it is plotting all 4 sensors and the Y axes of the other subplots is also wrong (please see output screenshot). I am also printing the array that contains the data ("print sep") just to check if the data is there.

Screenshot of the program output

Am I missing something obvious? Thank you very much for your help

tmdavison · Accepted Answer · 2017-07-24 16:32:00Z

1

When you create your 4 lines, they will all be created on the active figure (i.e. in this case, the last one you created).

You could move the creation of the 4 subplot axes to before your loop, and then create the matplotlib line instances on their correct axes (using the object-oriented ax.plot instead of plt.plot). Then, they will show up on the correct figures.

fig1 = plt.figure()
fig2 = plt.figure()
fig3 = plt.figure()
fig4 = plt.figure()

ax1 = fig1.add_subplot(111)
ax2 = fig2.add_subplot(111)
ax3 = fig3.add_subplot(111)
ax4 = fig4.add_subplot(111)

length = 20                             # Determines length of data taking session (in data points); length/10 = seconds

w = [0]*length                          # Create empty variable of length of test
x = [0]*length               
y = [0]*length
z = [0]*length

wline, = ax1.plot(w)                    # Sets up future lines to be modified
xline, = ax2.plot(x)                    
yline, = ax3.plot(y)
zline, = ax4.plot(z)

and then you can remove the following lines of code inside the for loop:

ax1 = fig1.add_subplot(111)
ax1.plot(wline.set_ydata(w))
...
ax2 = fig2.add_subplot(111)
ax2.plot(xline.set_ydata(x))
...
ax3 = fig3.add_subplot(111)
ax3.plot(yline.set_ydata(y))
...
ax4 = fig4.add_subplot(111)
ax4.plot(zline.set_ydata(z))

You will also need to change each plt.draw() to

fig1.canvas.draw()
fig2.canvas.draw()
fig3.canvas.draw()
fig4.canvas.draw()

and the plt.grid(True) should change to:

ax1.grid(True)
ax2.grid(True)
ax3.grid(True)
ax4.grid(True)

edited Jul 24, 2017 at 16:32

answered Jul 24, 2017 at 15:15

tmdavison

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3 Comments

Denis Over a year ago

thank you very much, that effectively solved the problem. I also fixed the scale (ylim) by applying the same reasoning (for example using ax1.set_ylim() instead of plt). Another issue came up though: Only the active figure is updating "its" plot in real time, i.e., by clicking/selecting one particular figure, that plot is being updated with the coming data but the 3 other figures are frozen unless I click/select them. Any idea? Thanks again.

tmdavison Over a year ago

Ah, that's because plt.draw only acts on the current figure. You should switch those to fig1.canvas.draw(), etc. See my edit above.

Denis Over a year ago

I was editing my previous comment. Thanks @Tom, it works. All plots updating correctly.

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