Plotting data with timestamps

Hi,

I have some data from a sensor and corresponding timestamps. How do I plot this data? (x axis - timestamps, y-axis - data) plot function gives empty figures. Probably because timestamps cannot be put in a linear scale. I tried semilogx - this also gives empty figures.

How do I do this?

Thanks in advance

[Ryan Smith]

Can you provide the data as an attachment?

[lexi11]

Hi, the timestamps are in unix format. I have attached a set of them. Since in excel it shows as 1.48E+10 format, I have changed the type in excel to 'number' so that you can see the entire number. So it adds .00 in the end to each value, but originally I get in the 1.48E+10 like number representation. Thank you.

Answer 1

[dpb]

Convert from whatever format the timestamps are in to a Matlab representation of dates...with R2014 up, there's the datetime class; prior to that use datenum. See <datetime> for current versions; plot is datetime -aware.

[Walter Roberson]

datetime() offers a 'ConvertFrom', 'posixtime'. The numbers you showed in the image convert into dates today if you divide them by 1000 before doing the conversion.

[lexi11]

This works!!! Thank you very much, dpb and Walter.

[lexi11]

Hi Walter, I would also like to know why we divide by 1000? Is this a standard procedure? Thanks.

[Munshi Mahbubur Rahman]

what have you guys used for the dateType argument? My data is in unix timestamp

[dpb]

Well, Unix timestamp is in seconds from Jan 1, 1970 but your data are too big for that; they seem to have been encoded as integer values to the millisecond resolution...

>> t=datetime(1489337264031/1000,'ConvertFrom','posixtime')
t = 
  datetime
   12-Mar-2017 16:47:44
>> t.Format=[t.Format '.SSS']
t = 
  datetime
   12-Mar-2017 16:47:44.031
>>

You'll notice the fractional seconds match up with the last significant digits in the original number.

[Peter Perkins]

In recent versions of MATLAB, you can also convert directly from ms since 1970:

>> datetime(1489337264031,'ConvertFrom','EpochTime','TicksPerSecond',1000,'Format','dd-MMM-yyyy HH:mm:ss.SSS')
ans = 
  datetime
   12-Mar-2017 16:47:44.031

The 'Epoch' parameter defaults to 1-Jan-1970, which is convenient if you have posix timestamps.

[dpb]

Thanks for making aware of that, Peter...