how to plot double against datetime

I am trying to plot a .csv file.

filename='data.csv'; 
data = readtable(filename);  % read csv file
time = data{:,1}+data{:,2};
X = data{:,3};
Y = data{:,4};
Z = data{:,5};
plot(X)           % OK
plot(time,X)      % NOT OK

When I plot X i get the figure that i want (see 1), but when I plot X against time, I get figure 2. Any ideas why this is happening and how to plot X against time and have the figure look like fig.1 with time in x axis?

Answer 1

[Clayton Gotberg]

In the attached CSV, the time data is the same for every 100 columns or so. Because of this, MATLAB is plotting all of the events that happened at that time at the same position on the x-axis. You will not be able to use the time information in the csv for plotting because the time is only measured to the second but you are taking measurements hundreds of times per second.

If you want to plot vs. time, you will need to create a new time vector.

time_new = linspace(start_time,end_time,size(X,1));
%start_time is the earliest time in the sequence

will create equally spaced points in time to plot X against. Note that if you started or stopped measuring in the middle of a second, the points will not match up with the true time exactly if you start or stop from the recorded value:

true_time = [0.5 0.75 1 1.25 1.5 1.75 2 2.25];
time_new = [0  0.2857  0.5714  0.8571  1.1429  1.4286  1.7143  2];

If the data samples you took are not equally spaced in time, you will need to re-measure your data with a more accurate clock.

[Ancalagon8]

I get an error: Undefined function or variable 'start_time'.

[Clayton Gotberg]

I'm sorry, I should have clarified that start_time and end_time would need to be defined by you before you ran this command.

start_time = % The earliest time for which you have data
end_time = % The time you stopped taking measurements

[Ancalagon8]

I defined

time_new = linspace('14:44:47','14:44:59',size(X,1));

and i get "inputs must be scalars" error message.

[Clayton Gotberg]

That is because '14:44:47' and '14:44:59' are characters, not scalars. You need to choose or determine scalars that fit the interval.

For example,

time_new = linspace(47,59,size(X,1));

should work and you can translate beetween one of the resulting values - say 53.75 - as meaning '14:44:53.75'.

If when you plot the resulting data you want the time to show up as a label on the X-axis, look at the XTickLabels command. @Star Strider's answer about labelling charts with characters should be a great start.

[Walter Roberson]

start_time = duration(14,44,47);
end_time = duration(14,44,59);
time_new = linspace(start_time, end_time, size(X,1));

[Ancalagon8]

Thank you!