How can I remove the date of a datetime matrix ?

Hello,

I am using a datetime matrix in the format which may be 'dd MMM yyyy HH:mm:ss' (first data is '25 Nov 2016 08:00:00' and last one is '25 Nov 2016 19:00:00'. I have some issues when I want to change the x-axis scale of my plots as every data is currently 7.3666e+05 when I use datenum and so I cannot really use it like this.

1) As every data is on the 25/11/16, I would like to use a 'time' matrix instead of a datetime matrix. This may help me with changing the x-axis scale if I can get much smaller numeric values than 7.3666e+05 with datenum.

Am I right ? And if yes, would you know how I could get this 'time' matrix ?

2) If this is not possible or if this cannot help me better, is there a way to change the x-axis scale using directly time with something like the followings ?

 xlim(['25 Nov 2016 12:18:00' '25 Nov 2016 12:23:00'])

or

 xlim(['12:18:00' '12:23:00'])

Thank you very much for your help.

Arnaud

Answer 1

[Jan]

Did you try datetick already? While the xlim limits require the data in the datenum format, the visible tick labels can be set to the date strings easliy with this command.

[Arnaud]

Hi, thank you for your answer. Yes I tried it but I may not use it right...

I have the first plot with HH:mm in the x-axis when I plot the 'datetime data' in x.

The second plot with 7.3666e+05 is when I plot datenum('datetime data') in x.

In that case, I cannot select another range of x-axis if I want to zoom in (e.g. between 12:10 and 12:20 in plot 1) as all x-axis values appear the same in plot 2...

Could you tell me how I should use datetrick in that case as I do not see how this could help me ? In my understanding, datetrick only allows to change the format of the x-axis and not the scale, am I right ?

Thanks, Arnaud

[Jan]

I do not understand this sentence:

 In that case, I cannot select another range of x-axis if I want
 to zoom in (e.g. between 12:10 and 12:20 in plot 1) as all x-axis
 values appear the same in plot 2...

In which case? How do you try to select the range? What does "I cannot" mean explicitly? What is "all x-axis value" and they are the "same" as what?

I still do not see, what your problem is. You are right: datetick changes only the tick labels and for setting the limits the command xlim is needed. xlim accepts the numerical date numbers only, and this is exactly what you have to do. So do not use:

xlim(['12:18:00' '12:23:00'])   % FAILS

but

xlim([datenum('12:18:00'), datenum('12:23:00')])

[Arnaud]

Thank you for your comment.

xlim([datenum('12:18:00'), datenum('12:23:00')])

works but according to the plot I got after this command, it seems to go to the right time but for another day as I got an empty plot with no data. And that day may be 01 Jan 2017 as when I did datenum('12:18:00') on the command window, I got 736696.5125 which is 01-Jan-2017 12:18:00... Would you know why ?

When I use

xlim([datenum('25 nov 2016 12:18:00'), datenum('25 nov 2016 12:23:00')])

it works well so thank you very much for your help !!

But in order to have it simpler, would there be a way without writing '25 nov 2016' as it would always be that same day ?

Thank you, Arnaud

Answer 2

[Peter Perkins]

It seems like the answer to this question is, "don't plot with datenums." Since R2014a, plotting with datetimes will already leave the date portion of the timestamps out of the tick labels if the data spans only one day.

Plotting datetimes really does work better than using datenum and datetick in most cases, and since R2016b, in all cases. Unless you are using something older than R2014b, or unless you have some good reason not to, use datetime.

[Joshua Remba]

Hi Peter, My data spans only 1 day, and the date (month date, year) is written a single time in the bottom right hand corner, below the HH:mm labeled ticks. How can I get rid of that date entirely? Thanks in advance!

[Peter Perkins]

If that's really what you want, convert the datetimes to durations using the timeofday function.