plot with two datetick axis

I want to plot x,y data where the x axis is a datenum. And I want to display the x-axis details in both UTC time and local time, which is 6 hours later. Datetick seems to want to create tick marks at "good" places for time so humans can make better sense of it. But having both time zones represented means the data seems to be shifted 6 hours.

So, is it possible to have two x-axis of tick marks and labels each of which is for the same x,y data but one is shifted 6 hours.

Thanks,

RAndall

Answer 1

[dpb]

Don't use datenum and datetick, use datetime instead for which plot has builtin support.

You should be able to create two independent x-axes but plot against only the one--it's unfortunate TMW didn't complete the task when they built yyaxis with its twin xxaxis that would make things more user-friendly.

Answer 2

[Adam Danz]

dpb's solution is a good one. You'll need to make sure the upper and lower tick marks align (use 'XTick' property and xlim).

Here's an alternative that puts a 2nd line of xlabels along the bottom axis. Since TMW doesn't allow multiple lines in the axis labels, this replaces the XTickLabel with text objects.

% Create figure
figure
t = hours(5):minutes(30):hours(8);
y = rand(1,7);
plot(t,y);
xtickformat('hh:mm')
axh = gca(); 

% Set x tick and x lim.
% It's important theses are set and not 'auto'
xlim([min(t), max(t)])
axh.XTick = t;

% compute time in other time zone
tShift = t - hours(3);  %assuming 3 hr difference

% Create the tick labels
ticktxt = join([string(datestr(t,'hh:MM')),repmat(newline,numel(t),1),datestr(tShift,'hh:MM')]);

% add new tick labels as text objects 
axh.XTickLabel = [' ']; 
text(axh.XTick, min(ylim(axh))*ones(size(axh.XTick)), ticktxt, ...
    'HorizontalAlignment', 'center', 'VerticalAlignment', 'Top'); 

% add axis lables
xlh = xlabel('Local time / UHC (-3)');  %bottom
xlh.Position(2) = xlh.Position(2)-.008;  %move the xlable down a bit

[dpb]

Out of curiosity, there seems to be a slight alignment issue on the lower of the two??? Is there an extra space in there or something, maybe??

[Adam Danz]

I noticed that too. The following line produces the x tick labels.

% Create the tick labels
ticktxt = join([string(datestr(t,'hh:MM')),repmat(newline,numel(t),1),datestr(tShift,'hh:MM')]);

the string() command does not add extra space.

datestr(t,'hh:MM')
ans =
  7×5 char array
    '05:00'
    '05:30'
    '06:00'
    '06:30'
    '07:00'
    '07:30'
    '08:00'

It appears that the newline() command may be adding space before and after the char(10) (newline) character. Note the space before and after the return carriage.

ticktxt = 
  7×1 string array
    "05:00 ↵ 02:00"
    "05:30 ↵ 02:30"
    "06:00 ↵ 03:00"
    "06:30 ↵ 03:30"
    "07:00 ↵ 04:00"
    "07:30 ↵ 04:30"
    "08:00 ↵ 05:00"
    
size(ticktxt{1})
ans =
     1    13 
     % size would be 11 otherwise

ADDENDUM

The space is added with strings (" ") but not characters (' ').

t = join(["1",newline(),"2"])

t = 
    "1 
      2"
      
t = join(['1',newline(),'2'])
t =
    '1
     2'