How to show a seconds plot axis ticks as hh:mm

[Edited to clarify that X is a 2x2, part of a meshgrid.]

Assume the data for the x-axis is duration in seconds, in both directions, with values ranging from -10800 to +-10800 seconds.

I use a contourf(X,Y,Z) to plot it.

In the x-axis, instead of:

-10800, -9000, ... , 9000, 10800

I would like to show the following ticks:

-03:00, -02:30, ... , +02:30, +03:00

That is:

• Change seconds duration to hh:mm
• Keep negative sign in front of the negative durations
• Add the plus sign to the positive values (I could do this via xtickformat('%+4.4g');, but I am not able to combine this with the hh:mm format)

I have been playing around with xtickformat and datetick, but had no success so far.

Thank you!

Answer 1

[Star Strider]

Try this:

X = -10800:10:+10800;
tv = seconds(X);

signal = sin(2*pi*X/1E+3);                          % Create Signal

figure
plot(tv, signal, '-')
xtickformat('hh:mm')

I created ‘signal’ to test the code. This works as desired in R2020a.

.

[Mitsu]

Thank you. I am afraid my question missed a lot of details... your code works perfectly, but when I try to apply it to a contourf (where X and Y form a meshgrid), I try this:

S_axis = seconds(X_axis);
contourf(S_axis, Y_axis, Z_axis, N)
xtickformat('hh:mm')

This returns the error: "Error using contourf - Input arguments must be numeric or objects which can be converted to double."

I also tried creating a separate variable for the xticks:

X_ticks = -10800:10:+10800;
X_ticks = seconds(X_ticks);
xticks(X_ticks);
xtickformat('hh:mm');

I obtain the following error when trying to use xticks: "Tick values must be a vector of increasing numeric values."

...and the following error if I don't convert it to a duration (comment second line): "Invalid numeric tick label format."

Do you have any advice on how to work around this?

[Mitsu]

My workaround for now is the following, but I wonder if there is a simpler way:

xt = xticks;
% Output
%  xt = -200  -150  -100   -50     0    50   100   150   200

xtl = {};
for i = 1:length(xt)
    dur = xt(i);
    if dur < 0
        dur = datetime('00:00:00') - minutes(dur);
        xtl{i} = strcat('-',datestr(dur,'HH:MM'));
    elseif dur > 0
        dur = datetime('00:00:00') + minutes(dur);
        xtl{i} = strcat('+',datestr(dur,'HH:MM'));
    elseif dur == 0
        xtl{i} = '0';
    end
end

% Output:
%  xtl = {'-03:20'}    {'-02:30'}    {'-01:40'}    {'-00:50'}    {'0'}    {'+00:50'}    {'+01:40'}    {'+02:30'}    {'+03:20'}

xticklabels(xtl)

[Star Strider]

The duration array will not work as an argument to meshgrid or contour. Use the usual numeric arrays, then use the appropriate parts of my code to plot the x-axis (‘S_axis’ here).

Try this:

X = -10800:10:+10800;
Z_axis = rand(1001,numel(X));
[S_axis,Y_axis] = meshgrid(X, (0:1000));

figure
contour(S_axis, Y_axis, Z_axis)
xt = get(gca, 'XTick');
xtl = compose('%s', duration(seconds(xt), 'Format','hh:mm'));
set(gca, 'XTick',xt, 'XTickLabel',xtl)

It works when I run it (R2020a). You may need to tweak it to work with your contour call, however I believe it will work without modification.

[Image Analyst]

But it takes forever. I did this to get it to run in a reasonable time:

X = -10800 : 1000 : +10800;

[Star Strider]

Ferran — I believe my way is simpler.

Image Analyst — I believe the contour is what takes forever. I have no idea what Ferran’s data or contour call are. I got the duration array to work, and produce the appropriate tick labels with reasonable efficiency.

.

[Mitsu]

Thank you! I will try your suggested approach as soon as I come home today and "report back"!

[Star Strider]

My pleasure!

My day is over at this point, so I will check back here after 10:00 UTC.

[Mitsu]

Thank you, Star Strider!

Your suggestion works as fast as mine, and is way simpler. The only thing I cannot seem to do is add the plus sign for positive durations (I was playing with the suggested %+ suggested in the documentation for compose).

Whenever that is not necessary, I am going to opt for your approach! Cheers

Here's what it looks like:

[Star Strider]

As always, my pleasure!

Change the ‘xtl’ and set lines to:

xtln = compose('%s', duration(seconds(xt(xt<=0)), 'Format','hh:mm'));
xtlp = compose('+%s', duration(seconds(xt(xt>0)), 'Format','hh:mm'));
set(gca, 'XTick',xt, 'XTickLabel',[xtln xtlp])

and the ‘+’ signs magically appear!

[Mitsu]

Thank you. I feel dumb for not having thought of just splitting it! Cheers!

[Star Strider]

As always, my pleasure!

No worries! Cheers!