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 Current time on x axis in matplolib
#1
I'm monitoring and plotting temperature and pressure and would like to do so over the last 24 hours. I am getting the data from an Arduino sending the temp and pressure via the serial port in the following format "17.4 , 30.2". I can read and plot the data no problem but I can't figure out how to have the x axis show the current time on the right and the last 24 hours to the left. I have looked in the forums and some help files and have figured out how to get the hour and minutes but when I try to use either of those as my Xlim max it doesn't show the data. I think the problem is my data is not time stamped but goes by a sample count. I've tried a few things but am getting no where. I assume I'm not the first to do this, no sense reinventing the wheel.

What I would like is to have the current time on the far right of the graph, divided in 4 hour major ticks labeled with that time, minor ticks just showing no time attached. If the elapsed time goes past 12:00 midnight then the hours will show 8:00 of the previous day.

here's my code
# Graphing Temperature and Pressure

import serial # Import Serial Library
import numpy # Import numpy
import matplotlib.pyplot as plt #import matplot lib
from drawnow import *
import time
from datetime import datetime
from matplotlib.dates import DateFormatter, AutoDateFormatter, AutoDateLocator

tempArray = []
PressureArray = []

ArduinoData = serial.Serial('com8', 115200) # Read from Com8, that Mega is connect to at 115200 baud
plt.ion() # tell matplotlib you want iteractive mode to plot live data
count =0 # counts number of times loop runs

# Read Current Time
Current_time = time.localtime() #Gets current date and time
hour = Current_time[3] #Current hour
minute =Current_time[4] #Current minute
    
def makePlot(): #create a function that makes our desired plot    
    plt.ylim(0, 50) # sets min/max on y axis on graph
    plt.title('Live Temp and Pressure') # add title to graph
    plt.grid(True) # turn grid on
    plt.ylabel('Temp F') # Y axis label
    plt.plot(tempArray, 'ro-', label='Degrees F') # plots temp array as red with dots and connecting line
    plt.legend(loc='upper left') # puts Y axis line legend at top left of graph
    plt2=plt.twinx() # creates a second plot from first for pressure
    plt.ylim(27, 32) # set limit of 2nd Y axis
    plt2.plot(PressureArray, 'b^-', label = 'Pressure in "Hg') # plot Pressure with blue triangles and connecting line
    plt2.set_ylabel('Pressure "Hg') # adds label to second Y axis, have to use the set_label because of seconf plot
    plt2.ticklabel_format(useOffset=False) # use for when pressure is in Pascals keeps large number on 2nd Y axis scale
    plt2.legend(loc='upper right') # puts 2nd Y axis line legend at top right of graph

                            
while True: # forever while loop
    while (ArduinoData.inWaiting() == 0): # Wait for data in serial port
           pass # do nothing
    ArduinoString = ArduinoData.readline() # read data as a full string
    dataArray = ArduinoString.split(' , ') # split full string into parts separated by a ,
    temp = float (dataArray[0]) # convert first element in array to a float
    Pressure = float (dataArray[1]) # convert second element in array to a float
    
    tempArray.append(temp) # load temperature Array
    PressureArray.append(Pressure) # load Pressure Array
    drawnow (makePlot) # Call drawnow to update the live graph
    plt.pause(0.00001) # short pause needed for drawnow
    count = count + 1 # increments loop counter
    if(count > 50): # test if loop counts are above 50, defines length of X axis
        tempArray.pop(0) # removes first element of temp array
        PressureArray.pop(0) # removes first element of Pressure array

Thanks
John
Quote
#2
Since you haven't saved time, you'll have to use the sample number as a linear tick on the x axis.
Or, if you know what time the measurements started and the interval between samples, then you can calculate the time.
for setting up and labeling x, y see: https://plot.ly/matplotlib/figure-labels/
Quote
#3
I'm sorry but I can't follow what you've asked me to do. This is all new to me. I went on the link you provided and tried adding some of the custom legend lines but I can't figure it out.

Could you give me an example to work with?

Could I save the time as I'm reading the data in, (make a double array or a separate array?) and then use that as my x axis? The Arduino doesn't know what time it is so I would have to grab the time from my PC as I read the data.

Thanks for your help
John
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