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Plot same data set on two different y axis scales

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Elizabeth Yeap
Elizabeth Yeap on 5 Feb 2020
Edited: Adam Danz on 6 Feb 2020
NOTE: Question has been updated.
Hello all,
I want to plot a matrix (37 x 10) represented by sum_ET10 with 2 different y axis scales; km^3/year (original unit) on y axis lhs & mm/year (new unit) on y axis rhs. My code is shown below; I have also included an image below. Currently, my graph is still incorrect. What I want to do is to have a single plot as opposed to the two plots that I'm getting
%% Getting a 37 x 10 matrix of sum_ET10 & sum_area
decade = 10;
sum_area = squeeze(sum(reshape(area,size(area,1),decade,[]),2)/10);
sum_ET10 = squeeze(sum(reshape(ET,size(ET,1),decade,[]),2)/10);
% Plot the results
x_decade = 1:10;
f1 = figure(1);
x1 = axes;
plot(x_decade,sum_ET10(1,:),'-ok');
% Query the YTicks
yt = get(x1,'YTick');
% Create new axis based on existing axis
x2 = copyobj(x1,f1);
% Set new axis transparent
set(x2,'Color','none')
% Remove XTicks
set(x2,'Xtick',[])
% Display the YTicks on the right side
set(x2,'YAxisLocation','right')
% Converting values to mm/year
mm_sum_ET10 = yt/2199000*1000000;
% Update labels
set(x2,'YTickLabel',mm_sum_ET10)
grid on;
xlabel(x1,'Decade');
ylabel(x1,'km^3/year');
ylabel(x1,'mm/year');
untitled.jpg
Any help is appreciated. This is my first time doing different scaled axes, so I'm in need of help.
Thank you.

  1 Comment

Adam Danz
Adam Danz on 5 Feb 2020
Regarding the error, look at the size of sum_area and the size of yt. The ./ notation means that your dividing element i of the first variable by element i of the 2nd variable. That requires both variables having the same number of elements.
Update following your update
You need to copy the axis before plotting the line. But still, this is a bad solution. use yyaxis.

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Accepted Answer

Adam Danz
Adam Danz on 5 Feb 2020
Edited: Adam Danz on 5 Feb 2020
You should be using yyaxis instead of overlaying a 2nd axes which can lead to all sorts of problems.
Demo:
figure()
yyaxis left
plot(rand(1,20)*1000);
yyaxis right
set(gca, 'ytick', 0:10:100, 'ylim', [0,100], 'Ycolor', 'k')
See the link I provided for examples of yyaxis plots.
update
Applying that to your code would look something like this. The important part is to make sure the y limits of both y axes are set so that ylim(axis1) and ylim(axis2) are merely a conversion of each other. See the two lines below with the tag #important where you should make sure you're scaling the axes correctly.
% Plot the results
figure();
yyaxis left
plot(x_decade,sum_ET10(1,:),'-ok');
set(gca, 'YLim', [min(sum_ET10(1,:)), max(sum_ET10(1,:))]) % #important
yyaxis right
% Converting values to mm/year
mm_sum_ET10 = yt/2199000*1000000; % I assume yt is a vector
set(gca,'YTick',mm_sum_ET10, 'YLim', [min(mm_sum_ET10), max(mm_sum_ET10)]) % #important
ylabel('mm/year');
% go back to left axes (main axes)
yyaxis left
grid on;
xlabel('Decade');
ylabel('km^3/year');

  4 Comments

Show 1 older comment
Adam Danz
Adam Danz on 5 Feb 2020
Do you see two plots in the demo i provided? No. yyaxis merely controls which vertical axis, left or right, you're working with. If you're having trouble implementing the solution show me the code and i can help straighten it out.
Elizabeth Yeap
Elizabeth Yeap on 6 Feb 2020
@Adam Danz, I implemented your code with an additional line below; and am getting the figure below; How do I get the y values on the right axis to be at the same position as the left axis values?
yt = get(gca, 'YTick'); % after yyaxis right
Adam Danz
Adam Danz on 6 Feb 2020
Looking better! Set the ytick and ylim properties correctly. See the 2nd #important tag I referred to. Insert the correct values there.
I don't have access to your variables so I can only guess at what's going on. The idea is that the y ticks of the 2nd axis should be a direct conversion of the y ticks of the first axis. Likewise, the y limit of the 2nd axis should be a direct conversion of the y limit of the first axis. There's no need to use YTickLabel since your tick labels are numeric.
It would look something like this
yyaxis left
ax1 = gca();
yt = get(ax1,'YTick');
yl = ylim(ax1);
% convert
ytConverted = yt/2199000*1000000;
ylConverted = yl/2199000*1000000;
yyaxis right
set(gca,'YTick',ytConverted, 'YLim', ylConverted])

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