Sinusoid plots, XTick

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Patrick Star
Patrick Star on 17 Mar 2011
I've plotted a couple periods of y=5*sqrt(2)*cos(2*pi*t+pi/4) and I would like to use XTick (preferably, if it works) to set the labels on the x-axis to be all of the values of t such that y is at a maximum, minimum, or zero. I could do it manually, but I need it to be generic (i.e. I could change the function and the labels would conform to it). It could do it if there wasn't a phase shift, but there is. Does anyone know how to do this? Thanks.
For clarification: This image is the plot as it is now: http://img684.imageshack.us/img684/7678/whatihaveb.png I drew vertical lines where I want tick marks in this image: http://img838.imageshack.us/img838/1121/whatiwant.png
  5 Comments
Patrick Star
Patrick Star on 17 Mar 2011
y will always be a sinusoid. If possible, I would like it to function at any frequency, but leaving it at a frequency of 1 Hz will be fine.
Walter Roberson
Walter Roberson on 18 Mar 2011
If you push the frequency high enough, the labels will overlap. Higher still and it will be a terrible mess to plot, when you start approaching one cycle per pixel.

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Accepted Answer

Paulo Silva
Paulo Silva on 17 Mar 2011
clf
t=0:0.01:2*pi;
y=5*sqrt(2)*cos(2*pi*t+pi/4);
%hold on %in case you want to draw your own y lines instead of using GRID
plot(t,y,'r'); 
mx=max(y); %find the maximum value of y
mxm=min(y); %find the minimum value of y
%draw your own lines 
% line(get(gca,'Xlim'),[mx mx],'LineStyle','--','Color',[0 0 0]);
% line(get(gca,'Xlim'),[-mx -mx],'LineStyle','--','Color',[0 0 0]);
% line(get(gca,'Xlim'),[0 0],'LineStyle','--','Color',[0 0 0]);
t1=find(y>=(mx-0.01)); %dirty way to find maximum
t2=find(y>=-0.23 & y<=0.23); %dirty way to find zeros
t3=find(y<=(mxm+0.01)); %dirty way to find minimum
ti=[t1 t2 t3]; %put all the interesiting values on a vector
ti=sort(ti); %put the values in ascending order
v=[find(diff(ti)~=1) numel(ti)]; %remove unwanted values and add last one
%set the ticks and turn on grids
set(gca,'XTick',t(ti(v)))
set(gca,'XTickLabel',t(ti(v)))
set(gca,'XGrid','on')
set(gca,'YTick',[mxm 0 mx])
set(gca,'YTickLabel',[mxm 0 mx])
set(gca,'YGrid','on')
axis([0 max(t(ti(v))) mxm-0.2*abs(mxm) mx+0.2*mx]) %adjust axis

More Answers (1)

the cyclist
the cyclist on 17 Mar 2011
This code will identify just the maximum (not the minimum or zeros), but you should be able to see how to generalize it.
A cautionary note: you'll see in the plot from my code that it does not identify the second occurrence of the true maximum of the sine wave. This is because when I have sampled at every 0.01 interval of x, it is not a maximum of y there. You may need to code around that in your own data.
x = 0:0.01:4*pi;
y = sin(x);
figure
plot(x,y)
set(gca,'YLim',[-2 2])
[ymax indexToYmax] = max(y);
xAtMaxY = x(indexToYmax);
set(gca,'XTick',xAtMaxY);
set(gca,'XTickLabel',num2str(xAtMaxY));
set(gca,'XGrid','On')

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