plotyy how to make yaxis align at zero

Hello everyone, I am using plotyy and it works fine so far. I was wondering if there is a way to align the two y axes at zero. So that I could plot a third line on top at zero for visual reference. The scale of both variables I am plotting is obviously different but the both range from negative values to positive one. Any ideas? Thank you so much in advance Sandra

Answer 1

[Geoff]

Well, if I plot some test data:

[ax, h1, h2] = plotyy([1 2 3], [-1 5 6], [2 3 4], [-2 1 -1]/2);

I get back two axis handles in ax, and also a handle to my two data series. I can set YLim on each of the axes individually. The question is how to adjust YLim such that the zeros both align...

The simplest approach is to put zero in the centre.

maxval = cellfun(@(x) max(abs(x)), get([h1 h2], 'YData'));
ylim = [-maxval, maxval] * 1.1;  % Mult by 1.1 to pad out a bit
set(ax(1), 'YLim', ylim(1,:) );
set(ax(2), 'YLim', ylim(2,:) );

But that's not usually a great idea. Your data is probably not symmetric about zero, and/or you might want to put zero in a specific place.

I haven't done this sort of thing for a while, and don't feel like scratching my head over it right now... But the upshot is now you know HOW, you just need to find a suitable formula to align the ranges such that the ratio of zero to the min and max value is the same on both axes.

[Sandra]

Thank you so much. That does the job perfectly and is all I needed!!!

YAAYY

Answer 2

[Zhonglu Lin]

a slightly better one based on Geoff's answer.

%%align zero for left and right
yyaxis right; ylimr = get(gca,'Ylim');ratio = ylimr(1)/ylimr(2);
yyaxis left; yliml = get(gca,'Ylim');
if yliml(2)*ratio<yliml(1)
    set(gca,'Ylim',[yliml(2)*ratio yliml(2)])
else
    set(gca,'Ylim',[yliml(1) yliml(1)/ratio])
end

[Toby Snaire]

Awesome - this helped me out too. Copy-paste, switch 'left' and 'right' in my case, and boom!

[Image Analyst]

There is a ylim() function you know. It's simpler and more direct than using the set() function. Of course I think my answer was the simplest and most direct to match the axes limts of the two different axes:

h = plotyy(x, y1, x, y2) % Gets two handles
h(2).YLim = h(1).YLim; % Set the right one equal to the left one.

[Sanders A.]

Image Analyst: but I have vastly differnt scales for my left and right sides so your method doesn't work.

[Image Analyst]

Can you prove it by starting a new question and attaching your script, data, and screenshot?

Answer 3

[Davide Fenucci]

This should work for all the possible combinations

function align_yyaxis_zero(ax)
    % align zero for left and right
    yyaxis left;  yliml = get(ax,'Ylim');
    yyaxis right; ylimr = get(ax,'Ylim');
    
    % Remove potential zeros from the limits
    yliml = yliml - 0.05 * (yliml == 0) .* yliml([2 1]);
    ylimr = ylimr - 0.05 * (ylimr == 0) .* ylimr([2 1]);
    
    if yliml(1) > 0 && ylimr(1) > 0
        yliml = [0 yliml(2)];
        ylimr = [0 ylimr(2)];
    elseif yliml(2) < 0 && ylimr(2) < 0
        yliml = [yliml(1), 0];
        ylimr = [ylimr(1), 0];
    elseif yliml(1) > 0 && ylimr(2) < 0
        ratio = diff(yliml)/diff(ylimr);
        yliml = [ylimr(1)*ratio, yliml(2)];
        ylimr = [ylimr(1), yliml(2)/ratio];

    elseif yliml(2) < 0 && ylimr(1) > 0
        ratio = diff(yliml)/diff(ylimr);
        yliml = [yliml(1), ylimr(2)*ratio];
        ylimr = [yliml(1)/ratio, ylimr(2)];
    elseif yliml(1) > 0
        yliml(1) = yliml(2) * ylimr(1) / ylimr(2);
    elseif yliml(2) < 0
        yliml(2) = yliml(1) * ylimr(2) / ylimr(1);
    elseif ylimr(1) > 0
        ylimr(1) = ylimr(2) * yliml(1) / yliml(2);
    elseif ylimr(2) < 0
        ylimr(2) = ylimr(1) * yliml(2) / yliml(1);
    else
        dl = diff(yliml);
        dr = diff(ylimr);
        if yliml(2)/dl > ylimr(2)/dr
            ylimr(2) = yliml(2)*dr/dl;
            yliml(1) = ylimr(1)*dl/dr;
        else
            yliml(2) = ylimr(2)*dl/dr;
            ylimr(1) = yliml(1)*dr/dl;
        end
    end

    yyaxis left;  set(ax, 'YLim', yliml);
    yyaxis right; set(ax, 'Ylim', ylimr);
end

Answer 4

[Image Analyst]

Try this:

% Sample data
x = 1 : 50
y1 = 2 * x - 30;
y2 = 100 * cos(x/50) - 70;
% Plot, getting back handles to two axes.
h = plotyy(x, y1, x, y2)
grid on;
% Make the x axis of axes #1 go through the Y=0 point.
h(1).XAxisLocation = 'origin';
% Make the y axis range of axes #2 match that of axes #1
% Meaning they will both share the same x axis and the
% x axis will go through the Y=0 point of both the left and right axis.
% OOP programming valid for R2014b and later.
h(2).YLim = h(1).YLim;

Answer 5

[Juan Miguel Serrano Rodríguez]

Another alternative using yyaxis:

% First plot
plot(ensayo.TimeStamp, ensayo.(ptop.medidas.Mprod.sensor_id)); 
ax = gca; hold on; 

ylabel("m3/h"); yyaxis right; ylabel("kg/s"); % Labelling

% Second plot
Mprod_kgs = ensayo.(ptop.medidas.Mprod.sensor_id) .* densSatLiqTW( ensayo.(ptop.medidas.Tprod.sensor_id) )/3600;
plot(ensayo.TimeStamp, Mprod_kgs); title("Evolución Mprod"); 

% Make the axis align
ax.YAxis(2).Limits = ax.YAxis(1).Limits; 

[Jun Liu]

that didn't work

[Mathias Magdowski]

This is a trivial answer. If both axis limits match, one usually does not need a plot with two y axes.