What does monotically increasing means?

I am trying to write 1 set of two subplots that outputs forecast predictions, but I keep getting this error

 ??? Error using ==> set
Values must be monotonically increasing

 Error in ==> borrame at 29
 set(gca,'XTick',[x(1) x(2) x(3) x(4) x(5) x(6) x(7)])

The code goes like this,

 ymin = [12 12 14 4 13 17 16];
 ymax = [17 17 18 17 18 22 21];

 ymin1 = [4 4 4 4 6 5 6];
 ymax1 = [4 4 4 5 6 6 8];

 x1 = floor(now) +7/24;% TODAY
x2 = floor(now + 0.50) +7/24;% TONIGHT
x3 = floor(now + 0.75) +7/24;% TOMORROW
x4 = floor(now + 1.50) +7/24;% TOMORROW_NIGHT
x5 = floor(now + 1.75) +7/24;% TWO_DAYS_FROM_TODAY
x6 = floor(now + 2.50) +7/24;% THREE_DAYS_FROM_TODAY
x7 = floor(now + 3.50) +7/24;% FOUR_DAYS_FROM_TODAY

 x = [x1,x2,x3,x4,x5,x6,x7];

 subplot(2,1,1);
plot(x,ymax,'-ks','MarkerFaceColor','k','LineWidth',2)
hold on 
plot(x,ymin,'--ko','MarkerFaceColor','k','LineWidth',2)

 datetick('x',1,'keepticks');
title('Wind(Knots) versus Time (dd-mmmm-yyyy)','fontsize',12,'fontweight','b','fontname','times');
xlabel('Time (dd-mmmm-yyyy)','fontweight','b','fontname','times');

 set(gca,'XTick',[x(1) x(2) x(3) x(4) x(5) x(6) x(7)])
ylabel('Wind (Knots)','fontweight','b','fontname','times');
legend('Max','Min',2);
set(gca,'fontname','times')  % Set it to times
grid minor

 subplot(2,1,2);
plot(x,ymax1,'-ks','MarkerFaceColor','k','LineWidth',2)
hold on 
plot(x,ymin1,'--ko','MarkerFaceColor','k','LineWidth',2)

 datetick('x',1,'keepticks');
title('Waves (ft) versus Time (dd-mmmm-yyyy)','fontsize',12,'fontweight','b','fontname','times');
xlabel('Time (dd-mmmm-yyyy)','fontweight','b','fontname','times');

 set(gca,'XTick',[x(1) x(2) x(3) x(4) x(5) x(6) x(7)])
ylabel('Waves (ft)','fontweight','b','fontname','times');
legend('Max','Min',2);
set(gca,'fontname','times')  % Set it to times
grid minor

What I want is one set of two plots, and on the x-axis the variables be in this format,

 x1 = datestr(floor(now) +7/24, 'ddd dd/mmm')% TODAY
x2 = datestr(floor(now + 0.50) +7/24, 'ddd dd/mmm')% TONIGHT
x3 = datestr(floor(now + 0.75) +7/24, 'ddd dd/mmm')% TOMORROW
x4 = datestr(floor(now + 1.50) +7/24, 'ddd dd/mmm')% TOMORROW_NIGHT
x5 = datestr(floor(now + 1.75) +7/24, 'ddd dd/mmm')% TWO_DAYS_FROM_TODAY
x6 = datestr(floor(now + 2.50) +7/24, 'ddd dd/mmm')% THREE_DAYS_FROM_TODAY
x7 = datestr(floor(now + 3.50) +7/24, 'ddd dd/mmm')% FOUR_DAYS_FROM_TODAY

Can you please help?

Answer 1

[Star Strider]

I suggest calculating your dates ( x vector ) this way:

ofst = [0.0; 0.5; 0.75; 1.50; 1.75; 2.50; 3.50]*24;
xnow = floor(now);
for k1 = 1:length(ofst)
    x(k1,:) = addtodate(xnow, ofst(k1)+7, 'hour');
end
xchk = [x  datevec(x)]  % Optional, to be sure the dates are what you want

then to get the format in your datetick that you want, substitute this line:

datetick('x','ddd dd/mmm','keepticks');

for the datetick lines in your the code you posted. You don't need to change anything else.

Answer 2

[Matt Fig]

Monotonically increasing means that each value is larger than the previous value.

Yes = [1 2 3 4 5 6]
No = [1 1 3 4 5 3]

If you do this:

D = diff(x)

you will get this result:

D == [1 0 1 0 1 1]

Since not every element of D is positive, x is not monotonically increasing.

Try this:

x1 = floor(now) + 7/24;% TODAY
x2 = x1 + .5;% TONIGHT
x3 = x1 + 1;% TOMORROW
x4 = x1 + 1.50;% TOMORROW_NIGHT
x5 = x1 + 2;% TWO_DAYS_FROM_TODAY
x6 = x1 + 3;% THREE_DAYS_FROM_TODAY
x7 = x1 + 4;% FOUR_DAYS_FROM_TODAY
x = [x1,x2,x3,x4,x5,x6,x7];

Now have a look:

datestr(x,0)

ans =

10-Oct-2012 07:00:00
10-Oct-2012 19:00:00
11-Oct-2012 07:00:00
11-Oct-2012 19:00:00
12-Oct-2012 07:00:00
13-Oct-2012 07:00:00
14-Oct-2012 07:00:00

[Daniel Shub]

I think technically you gave the definition for strictly increasing, while the definition of monotonically means that each value is not smaller than the previous value.

[Matt Fig]

Daniel, I think you are correct for the mathematical definition, but not for the MATLAB definition.

close all
set(gca,'xtick',[0 .1 .2 .3 .4 .5])% No problem
pause(1)
close all
set(gca,'xtick',[0 .1 .2 .3 .3 .5])% Problem, and error...

[Star Strider]

Matt, PlanetMath seems to agree with you (and MATLAB).