Bokeh Streaming Plot is Blank

Question

My Bokeh Streaming Plot is a blank rectangle. I am able to create a simple line plot that does not update itself in real-time. I have read the Bokeh documentation for the version of Bokeh I am using 0.12.10 and Python3.5.3. I have searched on-line extensively for a solution to the error message.

I am getting the error

Error thrown from periodic callback: ValueError('All streaming column updates must be the same length')

I am using pyserial to retrieve data from a sensor. The example values are 73.40 for temperature and 12:30:42 for the time. I want to plot this data in real-time.

Here is the code:

import serial
from bokeh.io import curdoc
from bokeh.models import ColumnDataSource
from bokeh.plotting import figure, output_file, show

ser = serial.Serial('/dev/ttyACM0', 9600)

source = ColumnDataSource(dict(time=[], sensor=[]))

p = figure(plot_height=400, plot_width=1200, title="Fahrenheit Plotting")

p.title.text = "Fahrenheit Plotter"
p.title.text_color = "blue"
p.title.text_font = "arial"
p.title.text_font_style = "bold"
p.yaxis.minor_tick_line_color = "yellow"
p.xaxis.axis_label = "Time"
p.yaxis.axis_label = "Fahrenheit"



p.line(x='time', y='sensor',line_width=3,color="blue",alpha=0.8,source=source)


def update():
    while True:
        arduinoString = ser.readline()
        data_array = str(arduinoString).split(',')
        time = data_array[1]
        sensor1 = data_array[2]
        print(sensor)
        print(time)
        new_data = dict(time=[], sensor1=[])
        new_data['time'] = data_array[1]
        new_data['sensor'] = data_array[2]
        source.stream(new_data, 20)

curdoc().add_root(p)
curdoc().add_periodic_callback(update, 100)
curdoc().title = "Device Temperatures"

bigreddot · Accepted Answer · 2017-12-04 18:29:14Z

1

The example code is not self-contained, so I can't run it to fix, and reply with an updated version. But the error message is telling you what is wrong. All columns in a ColumnDataSource data dictionary must always be the same length at all times. You new_data must look something analogous to this:

new_data = {
    'time'  : [1,2,3,4],
    'sensor : [100, 200] 
}

The columns for time and sensor (which happen the be lists here, but could be arrays, etc) do not have the same length. That is the problem. All columns of a CDS must always have the same length, at all times.

answered Dec 4, 2017 at 18:29

bigreddot

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1 Comment

tluafed Over a year ago

Thank you. I made the temperature string and the sensor string the same length like this: 00072:94 12:55:33. I do not get the error message anymore.

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