MATLAB: How to place a legend on a plot produced from the PLOTYY command in MATLAB 7.5 (R2007b)

axislegendMATLABmultipleplotyy

For example, if I execute the following commands, the legend only contains one line and the figure contains only one line:

x = 1:10;
y = 10:-1:1;
[ax,h1,h2] = plotyy(x,x,x,y);
[legh, objh] = legend('str1', 'str2');

Best Answer

This enhancement has been incorporated in Release 2008a (R2008a). For previous product releases, read below for possible workarounds:

The PLOTYY command creates two axes. In the first axes it plots the first (x1,y1) data set and in the second axes it plots the second (x2,y2) data set.

With the code above, two axes are created and two lines are plotted. The first axes has a white color and the second axes has a color of 'none'. This allows you to see the line drawn in the first axes. When the LEGEND command is executed it obtains the line information from the current axes which contains only one line. Therefore, the resulting legend only shows one line.

To create a plotyy figure with a legend, try the following:

% Create the plotyy figure.  The handle to the first line is h1 and the handle 
% to the second line is h2.
x = 1:10;
y = 10:-1:1;
[ax,h1,h2] = plotyy(x,x,x,y);
% Make the second axes current (the axes with a color of 'none') so that both lines 
% are visible
axes(ax(2))
%Create the legend and explicitly state the handles to each line
[legh, objh] = legend([h1 h2],'str1', 'str2');

Related Solutions

MATLAB: Legend for yyaxis at two different locations

The second call to legend() is replacing the first legend (you can only have 1 legend).

I reworked your code to use yyaxis instead of plotyy.

Solution 1: use 1 legend

This solution puts all handles into the same legend. See inline comments for details.

n=-180:180;
x=(pi/180).*n;
y=sin(x);
y2=sin(x).*cos(x);
y3=cos(x);
% Plot on left axis

yyaxis left
h1 = plot(n,y, '-k', 'LineWidth', 1, 'DisplayName', 'h1'); 
set(gca, 'YColor', 'k') % set y axis color to same as line color



hold on
h3=line(n,y3,'Color','r','LineWidth',2, 'DisplayName', 'h3');
% Plot on right axis

yyaxis right
h2 = plot(n,y2, '-b', 'LineWidth', 1.5, 'DisplayName', 'h2'); 
set(gca, 'YColor', 'b') % set y axis color to same as line color

Solution 2: kludge in a 2nd legend

Since you can only have 1 legend per axis, this solution produces a second invisible axis and copies the h2 object to the invisible axis so that it can have its own legend. See inline comments for details.

n=-180:180;
x=(pi/180).*n;
y=sin(x);
y2=sin(x).*cos(x);
y3=cos(x);
% Plot on left axis
yyaxis left
h1 = plot(n,y, '-k', 'LineWidth', 1, 'DisplayName', 'h1'); 
set(gca, 'YColor', 'k') % set y axis color to same as line color
hold on
h3=line(n,y3,'Color','r','LineWidth',2, 'DisplayName', 'h3');
% Plot on right axis
yyaxis right
h2 = plot(n,y2, '-b', 'LineWidth', 1.5, 'DisplayName', 'h2'); 
set(gca, 'YColor', 'b') % set y axis color to same as line color
% Produce left-axis legend
legend([h1,h3], 'Location', 'NorthWest')
% Create invisible axis in the same position as the current axes
ax = gca(); % Handle to the main axes
axisCopy = axes('Position', ax.Position, 'Visible', 'off');
% Copy objects to second axes
hCopy = copyobj(h2, axisCopy); 
% Replace all x values with NaN so the line doesn't appear
hCopy.XData = nan(size(hCopy.XData)); 
% Create right axis legend
legend(hCopy, 'Location', 'NorthEast')

MATLAB: How to plot two plotyy plots in the same plot

[ax,h1,h2] = plotyy(...) 
hold(ax(1))
plot(ax(1),...)
hold(ax(2))
plot(ax(2),...)

The problem was simple, hold on just holds the current axes and your plotyy makes two axes so only one is the current.

Best Answer

Related Solutions

MATLAB: Legend for yyaxis at two different locations

MATLAB: How to plot two plotyy plots in the same plot

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