Repeating time values on x-axis

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Noush 2021-11-23

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评论： dpb 2021-11-30

Hi there,

I have a table that contains a first column with time of the day and a second with the power of a machine. I would like to plot the power on the y-axis and the time on the x-axis for a week.

When I use the command plot(time,power) I get this:

The graphs overlap, as the times repeats itself for each day. I don't want that. I dont want to use the xtick method to solve this, as it would require me to specify each xticklabel right? That would take too long. Rather, I'd like the graph to produce the xticks automatically and plot in such a way that the graphs for each day are lined up after each other.

I also have a column with the dates, I just don't know how to put that all together in the correct way for it to plot it the way I want.

Can you help?

Thank you!

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dpb 2021-11-30

编辑：dpb 2021-11-30

在 MATLAB Online 中打开

The problem in your original table and what Peter was explaining is that you convert Zeit into a datetime instead of a duration variable. A datetime ALWAYS includes a date even if the format to display it hides it as you do with your input conversion.

To illustrate--

>> zeit=datetime('0030','InputFormat','HHmm','Format','h:mm a')
zeit = 
  datetime
   12:30 AM
>> datestr(zeit)
ans =
    '30-Nov-2021 00:30:00'
>> 

shows that even though the display value on the screen looks like just a time of day, it really is a full-fledged date AND time inside. All operations on the variable zeit other than the output of the representation on the screen use the full internally stored value of the variable; the display format is simply that--just what you see, not what the value actually is. It's no different than showing only the integer portion of a floating point variable by using an integer format expression with a double precision variable...

>> p=pi; fprintf('%.0f\n',p)
3
>>

Above you recognize the the variable p still == pi internally, you've just not displayed the fractional portion. That's what you've done with the time variable -- just hidden the implicit day value that goes along with the variable by specifying a format string to use to display the result.

The difference between numeric and time-related variables is that you have to make the format explicit when you display numeric values other than the default setting whereas time variables carry a format along with them inherent in the time variable object. This can, agreed, be confusing.

Secondly, by using a datetime object for zeit, you made it such that you can't add the "time"(*) to the date because math operations other than ordinal comparison aren't defined...you can't add two datetimes. As Peter showed more directly and as you did above, you CAN (and must) add a duration to a datetime to get a datetime with both a date and time.

(*) And, of course, it wouldn't make any sense to add the datetime value of the zeit variable to the date, anyway, which is why the operation is undefined.

You can get to your same location as above with

dt=datetime(T(C:B,1),'InputFormat','yyyyMMdd')+duration(T(C:B,2),'InputFormat','HHmm');

using the input data themselves without the intermediaries.

Or, in all likelihood if the data come from an input file originally, then you could use the import options object with readtable and read them in as datetime and duration arrays directly.

To set the plot tick format, use Peter's hints above

hXAx=get(gca,"XAxis"); % return the handle to the x-axis

which will be a datetimeruler object. You can set the format string for it to anything allowable for a datetime -- the problem you'll likely run into is that there's limited room on the axis for date strings so when you try to add both dates and time they'll probably get too crowded to read. Also you only get tick labels where there are ticks and the default appears to have placed just one tick per day. Again, you can change that by setting the XTicks property to a series of datetime values of your choosing, but you're going to run out of room real fast for display.

dpb 2021-11-30

编辑：dpb 2021-11-30

在 MATLAB Online 中打开

Oh,yes. I just had a discussion w/ TMW on the limitations in the duration object regarding input formats this past week and forgot already again it has overly restrictive string formatting input requirements.

You can try something like

duration(datestr(datetime(T(C:B,2),'InputFormat','HHmm','Format','h:mm a'),'HH:MM'),'InputFormat','hh:mm')

and see if it will work for you. That's going through the datetime object and using datestr to reformat the string into a recognizable form. In theory, it should work, but we all are aware of the difference between theory and... :)

If it doesn't succeed, the simplest workaround will be to introduce a temporary variable in between to extract the [h,m] vectors from the datetime and then pass that to duration

zeit=datetime(T(C:B,2),'InputFormat','HHmm','Format','h:mm a'); % go ahead with datetime first
[h,m]=hms(zeit);                                                % get hr, min from it
dt=datetime(T(C:B,1),'InputFormat','yyyyMMdd')+duration(h,m,0); % create full datetime

If you would attach a section of the actual data file, folks could play with the real thing...

dpb 2021-11-30

在 MATLAB Online 中打开

The other markedly different alternative would be to catenate the two variables as text and then scan them as a composite datetime...

dt=datetime(strcat(T(C:B,1),T(C:B,2)),'InputFormat','yyyyMMddhhmm');

which may well be the shortest, best way in the end.

NB: the above input format string will require the times all have double-digit hours, not single. The one-letter form of 'h' format string only works if the hour and minute fields are separated with the colon or other punctuation; it's simply too ambiguous a format without that indicator.

dpb 2021-11-30

"... convert text to datetime ... then immediately call timeof day on that."

Ah! I don't recall having come across that suggestion before, Peter, but it's a good one. I didn't even think of timeofday ...

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