plot with two axes and two legends

Hello Dear Matlab community,

How can I plot two sets of data using two y-axes with a common x-axis, adding two legends, one legend for each set?

The 2 straight lines belong to the y-axis right, while the 4 curves belong to the y-axis left. I can succesfully plot everything until I reach the legends (see my code, which used to work until 2012b, not sure?). When I execute hl1 plots the legend for the 1st axes (OHM1...OHM4), but by executing hl2 deletes the previous legend and it draws the second legend, obviously , I would like to keep both. The picture shows the desired result. I used plotyy, however, I get a hint to use "yyaxis" still stuck :(

Thanks

Julia

hf = figure;               
[ax,p1,p2] = plotyy(t,volt,t,Inte_p);    

set(get(ax(1),'Ylabel'),'String','AC (V.)');
set(get(ax(2),'Ylabel'),'String','I (A)');
y1max = max(get(ax(1),'YTick'));
set(ax(1),'YTick',0:y1max);
set(ax(2),'YTick',0:20:100);  
hl1=legend(ax(1),p1(:),{'OHM1';'OHM2';'OHM3'});
hl2=legend(ax(2),p2(:),{'Int1'; 'Int2'});

Answer 1

[Jaswanth]

Hi,

To create a figure with two sets of y-axes sharing a common x-axis, you can use the MATLAB function yyaxis. This function specifies the active side for the axes instead of the current axes. If the axes do not already include two y-axes, this command adds a second y-axis.

In MATLAB, the legend function allows only one legend per axes. To work around this limitation, you can follow these steps:

• Create a legend for the first set of plot objects.
• Create a new set of axes `a` with the same position as the current axes but turn off its visibility.
• Create another legend referencing the next set of plot objects and associate it with the invisible axes `a` to avoid conflicts with the first legend.

Please refer to the following example code demonstrating the implementation discussed above:

hf = figure;

% Define your data
t = 0:0.1:10; % Example x-axis data
volt = [sin(t); cos(t); sin(2*t); cos(2*t)]; % Example y-axis data for left y-axis
Inte_p = [1.5*t; 2*t]; % Example y-axis data for right y-axis

% Define colors
colors = lines(6); % Generate a set of colors

% Create left y-axis plots
yyaxis left
p1(1) = plot(t, volt(1,:), 'Color', colors(1,:)); hold on;
p1(2) = plot(t, volt(2,:), 'Color', colors(2,:));
p1(3) = plot(t, volt(3,:), 'Color', colors(3,:));
p1(4) = plot(t, volt(4,:), 'Color', colors(4,:));
ylabel('AC (V.)');
yl = max(ylim);
yticks(0:yl);

% Create right y-axis plots
yyaxis right
p2(1) = plot(t, Inte_p(1,:), 'Color', colors(5,:), 'LineStyle', '--'); hold on;
p2(2) = plot(t, Inte_p(2,:), 'Color', colors(6,:), 'LineStyle', '-.');
ylabel('I (A)');
yticks(0:20:100);

% Create the first legend
legend([p1(1), p1(2), p1(3), p1(4)], {'OHM1', 'OHM2', 'OHM3', 'OHM4'}, 'Location', 'southwest');

% Create new axes with visibilty off to avoid interference with the existing plot visually.
a=axes('position',get(gca,'position'),'visible','off');

% Create the second legend associated with the invisible axes a to avoid conflicts with the first legend.
legend(a, [p2(1), p2(2)], {'Int1', 'Int2'}, 'Location', 'northeast');

% Set common x-axis label
xlabel('Time (s)');

hold off;

Kindly refer to following MathWorks documentation to know more about the yyaxis function discussed above: https://www.mathworks.com/help/matlab/ref/yyaxis.html?searchHighlight=yyaxis&s_tid=srchtitle_support_results_1_yyaxis

I hope the solution provided above is helpful.