2

I run into this problem: when I plot in Octave without x values, it's ok. when I add x values (which are supposed to be dates, but I don't impose the date format), it goes weird

ab = rand(96,1);
close all
figure
plot(ab)

Gives this figure:

plot without x values

close all
figure
plot(datenum('01-Nov-2020 00:00','dd-mmm-yyyy HH:MM'):1/24/4:datenum('01-Nov-2020 23:45','dd-mmm-yyyy HH:MM'),ab)

Gives this figure: plot with x values As you can see, the x values are all within 1 day, so I don't even know why the x tick show more than 1 day...

Anyone would have a smart explanation?

Lucile
  • 93
  • 6
  • 1
    Hi Lucile, actually yes, I've seen this before. See here: https://stackoverflow.com/questions/53229703/legend-outside-plot-does-not-work-with-plotyy-in-octave/53242735#53242735 Unfortunately that question had a bit of a red-herring in the title, so it's hard to search for ... – Tasos Papastylianou Dec 16 '20 at 10:47

1 Answers1

4

EDIT: As Tasos stated in an earlier answer, it's a "problem of OpenGL being unable to deal with the precision involved when adding small numbers to a large number".

From my point of view, that's "just" an Octave bug, MATLAB (tested with MATLAB Online) doesn't show that behaviour. It seems, Octave has problems with small x intervals when the actual x value is relatively high, cf.

subplot(2, 1, 1);
x = linspace(800000, 800000 + 2*pi, 100);
y = sin(x);
plot(x, y);

subplot(2, 1, 2);
x = linspace(0, 2*pi, 100);
y = sin(x);
plot(x, y);

The corresponding output is:

Output sine

It seems, some x values are getting "merged"!? I haven't searched in depth for reported bugs on that issue...

But, to work around that issue in your case, you could try plotting the data starting at 0, and later just adapt the xticklabels:

% Data
ab = rand(96, 1);

figure(1);

% Plot without dates
subplot(3, 1, 1);
plot(ab);
xlim([1, 96]);

% Plot with dates (standard, faulty)
subplot(3, 1, 2);
dates = linspace(datenum('01-Nov-2020 00:00', 'dd-mmm-yyyy HH:MM'), 
                 datenum('01-Nov-2020 23:45', 'dd-mmm-yyyy HH:MM'), 
                 size(ab, 1))
plot(dates, ab);
datetick('x', 'dd-mm-yyyy HH:MM');
xlim([dates(1), dates(end)]);

% Plot with dates (custom, correct)
subplot(3, 1, 3);
# For plotting, subtract start date
dates_for_plot = dates - datenum('01-Nov-2020 00:00', 'dd-mmm-yyyy HH:MM')
plot(dates_for_plot, ab);
datetick('x', 'dd-mm-yyyy HH:MM');
# Add custom xticklabels with re-added start date
xlim([dates_for_plot(1), dates_for_plot(end)]);
xticklabels(cellstr(datestr(xticks + datenum('01-Nov-2020 00:00', 
                                             'dd-mmm-yyyy HH:MM'))));

That'd be the output:

Output data

HansHirse
  • 18,010
  • 10
  • 38
  • 67
  • See the link I posted in the comments. This has come up before :) – Tasos Papastylianou Dec 16 '20 at 10:48
  • @TasosPapastylianou Argh, haven't found that! (Even the workaround is the same.) Obviously a duplicate then. :-) – HansHirse Dec 16 '20 at 10:50
  • Well, kinda, you've isolated more or less the same problem / solution, but it may be worth keeping both questions, since the other one started off weird; this seems like a more targeted question / answer. I'd leave it, and keep the link in case it offers complementary details. :) – Tasos Papastylianou Dec 16 '20 at 10:52
  • Awesome! it works. cheers! yes sorry I had searched for the same problem but obviously didn't find it. – Lucile Dec 16 '20 at 11:02