I get date info when plot a Datetime in x-axis.

Hi, I am trying to plot a voltage data vs time data (HH:MN:SS) from a Datetime, but when I build the graph the x-axis always take the Year data (Dec 31, 1899).

I have tried modified the Datetime and the axis parameters, but always get this info.

How can I only display Hour:Minutes:Seconds? The year info is not important for me.

Thanks.

[the cyclist]

Can you upload your data, or a small subset, so that we don't have to create out own dataset (which also might differ in some important way from yours).

[Victor Olivero]

Thanks for your reply, I have solved the issue taking, for example, plot(timeofday(Time2),V_bateria) using the timeofday fuction. However, I will try your recomends.

[Victor Olivero]

My data come from a excel file, I import this to Matlab and after convert to a Datetime structure, then I get the hh:mm:ss but when I plot this data vs I_bateria or V_bateria, for example, the X-axis show the year data too, but it is a incorrect info (Dec 31, 1899).

Time	I_bateria	V_bateria	T_bateria
0.4540	9.2688	13.1237	25.5063
0.4543	9.2659	13.1725	25.3310
0.4547	9.5520	13.1481	25.4186
0.4551	9.5424	13.1481	25.5063
0.4554	9.5155	13.0992	25.0684
0.4558	9.5288	13.0992	25.1559
0.4561	9.5123	13.0748	25.0684

[dpb]

Yes, that too. A MATLAB datetime doesn't exist without an associated date and time.

Unlike the venerable and deprecated datenum or an Excel date that are both represented as a julian date in a double where the whole portion of the value represents days and the non-integer part the fractional portion of a 24-hour day, the datetime is an object containing both a date and a time and the constructor for it will create a date from thin air if one isn't provided.

timeofday() does the equivalent conversion to a duration object as the alternative suggested -- the difference is the subtraction of the origin of the timeseries I suggested will give you a 0 origin irregardless of the actual time, timeofday will return the actual hours of the day of the datetime values.

Which is to use if not just the fixup of the display of the datae depends on situation.

Answer 1

[dpb]

Another case of TMW being too aggressive in hiding things from the end user inside graphics objects -- the year/date 'SecondaryLabel' property is hidden and is shown by default for all datetime axes.

Use

hAx=gca;    % retrieve the current axes handle
hAx.XAxis.SecondaryLabel.Visible='off';

to make the year marker not be visible.

The alternative is to subtract the first time value of your datetime vector from all elements and convert to a duration and plot against it instead of the absolute time values.

dt=Time2-Time2(1);      % convert to elapsed time; is a ML duration, not a datetime
plot(dt,v_bacteria)

[Victor Olivero]

Thanks for your reply, I have solved the issue taking, for example, plot(timeofday(Time2),V_bateria) using the timeofday fuction. However, I will try your recomends.

Answer 2

[Adam Danz]

Starting in MATLAB R2023b, use the xsecondarylabel function to set, change, or remove a secondary label.

For more info and a demo, see this Graphics and App Building blog article.

https://blogs.mathworks.com/graphics-and-apps/2023/10/18/exploring-secondary-axis-labels-in-matlab/