MATLAB: Is it actually possible to use datetick

datenumdatetickdatetimexaxis

I have a cell array of timestamps with the format:

'2014-10-06 09:27:34'

I converted this to a datetime type using:

t = datetime(Date, 'InputFormat', 'yyyy-MM-dd HH:mm:ss');

Then, I made this into a DateNumber, so I could use it as the x-axis for a plot:

DateNumber = datenum(t);
plot(DateNumber, myData)

I tried to convert the x-asix to something readable using datetick. I only care about the hours, minutes and seconds , so I entered:

datetick('x', 'HH:mm:ss')

However, all my ticks say 00:10:00 no matter how I zoom in on the axis. Also, if I change the axis limits using the plot tool before I run datetick from the command line, it reverts to the old axis limits.

I tried plotting a small subset of my data (80 points) that was taken between 4:52:56 and 6:25:46. My plot only gets five ticks, 04:10:00, 05:10:00, 05:10:00, 06:10:00 and 06:10:00. How are these 'repeat' ticks possible?

If I look at the contents of DateNumber using datestr:

datestr(DateNumber(2550))
ans =
08-Oct-2014 04:52:56

I get back the values I expect, so it looks like the DateNumber variable is filled properly. The problems seem to be with datetick.

Best Answer

With datetimes, if you plot them it takes care of datetick for you:

plot(t)

It's a new feature in 14b and it will update when you zoom.

Related Solutions

MATLAB: Datenum Generating different numbers for same time difference

FP roundoff; datenum just uses a double for the time serial value; the integer portion is days and the fractional part the fractional portion of the day in seconds. It is susceptible to all the vagaries of fp arithmetic....

>> dn2=abs(datenum('01-04-2017 07:00:00','dd-mm-yyyy  HH:MM:SS')-datenum('01-04-2017 07:55:00','dd-mm-yyyy  HH:MM:SS'))
dn2 =
    0.0382
>> dn1=abs(datenum('01-04-2017 05:00:00','dd-mm-yyyy  HH:MM:SS')-datenum('01-04-2017 05:55:00','dd-mm-yyyy  HH:MM:SS'))
dn1 =
    0.0382
>> dn2-dn1
ans =
   1.1642e-10
>> ans*24*60*60
ans =
 1.0058e-05
>>

So the difference between the two values isn't zero but is about 10 usec or in absolute terms

>> dn=datenum('01-04-2017 05:00:00','dd-mm-yyyy  HH:MM:SS')
dn =
 7.3679e+05
>> eps(dn)
ans =
 1.1642e-10

or the closest can get to the difference in floating point for the size of the variable; iow, a double just can't be any more precise than the result obtained.

To see how close in minutes,

>> format longg
>> dn2*24*60
ans =
        55.0000000745058
>> dn1*24*60
ans =
        54.9999999068677
>>

Use the new-fangled datetime class instead; it's implemented as an object and has much more precision...

>> fmtDT='dd-MM-yyyy  HH:mm:ss';  % a little shorthand; the formatting is different, too...
>> dt1=abs(datetime('01-04-2017 05:00:00','inputformat',fmtDT)-datetime('01-04-2017 05:55:00','inputformat',fmtDT))
dt1 = 
  duration
   00:55:00
>> dt2=abs(datetime('01-04-2017 07:00:00','inputformat',fmtDT)-datetime('01-04-2017 07:55:00','inputformat',fmtDT))
dt2 = 
  duration
   00:55:00
>> dt1==dt2
ans =
  logical
   1
>>

and the two are identical.

There are many other enhancements with datetime as well that for almost everything now they are the preferred way to handle time.

MATLAB: (2016a) How to plot datetime versus numbers? Error: “Data inputs must match the axis configuration. A numeric axis must have numeric data inputs or data inputs which can be converted to double.”

For your first question, you've specified the format that datetime should use to interpret the representation of the time that you passed in, but you didn't specify the format in which the created datetime should be displayed. To do that, specify the 'Format' name-value pair argument (just like you specified 'InputFormat', with the desired format as the value in that pair.

dt1 = datetime('now')
dt2 = datetime('now', 'Format', 'yyyy-MM-dd HH:mm:ss')

For your second question, if you've previously plotted a line whose x data is numeric, you can't add another line to that same axes whose x data is a datetime.

x = 0:360;
y = sind(x);
plot(x, y)
hold on
x2 = datetime('today') + days(x);
plot(x2, y) % won't work

For this particular case, there's a reasonably intuitive mapping between x and x2. But in general there wouldn't be. In those cases answering a question like "Does x = 28 correspond to a week from now?" to line up the two plots could be difficult.

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