Error using legappend() - matlab

Question

I am writing a function that iterates through a loop and adds entries to a plot. When I try to use legappend(), though, I get the error below. I am passing it a string variable.
Error using legend>process_inputs (line 526)
Invalid argument. Type 'help legend' for more information.
Error in legend>make_legend (line 303)
[orient,location,position,children,listen,strings,propargs] =
process_inputs(ha,argin); %#ok
Error in legend (line 257)
[~,msg] = make_legend(ha,args(arg:end),version);
Error in legappend (line 74)
[legend_h,object_h,plot_h,text_strings] = legend(h,allDatah,str);
Here is a minimal example, taken from the MATLAB site
% Some data and old models:
x = (1:10)';
y = [x-5+x.^1.05 x-2 x-3 x-4 x-5];
% Plot the data and old models:
figure
plot(x,y(:,1),'mo','markersize',10);
hold on;
plot(x,y(:,2),'r');
plot(x,y(:,3),'b');
plot(x,y(:,4),'k');
plot(x,y(:,5),'kp');
box off
axis([1 10 -5 20])
legend('data','model 1','model 2','model 3','model 4','location','northwest')
legend boxoff
myNewModel = x - 5.5 + x.^1.1;
plot(x,myNewModel,'m','linewidth',2);
legappend('my new amazing model!')

Answer 1

As mentioned in my comment, MATLAB's graphics engine was significantly overhauled in R2014b. While it brought a plethora of very welcome changes, like any major code overhaul it broke functionality in the existing code base. Relevant here is implementing legends as their own object class rather than as a cobbled together axes object. Given its July 2014 release date, legappend was likely created using R2014a and the logic in the code assumes that the legend is an axes object. This unfortunately breaks in the new graphics system.
Fortunately, the fix isn't as complex as I was anticipating. If you take a look at the properties of the new legend object, there's no documented property for the linked data. Attempting to set the 'String' property manually also has no effect. However, if you look at the final syntax in the function description ([l,icons,plots,txt] = legend(___)), it seems clear that legend has a way to access the appropriate internal properties. And indeed, if you poke around in the legend source, you'll find the 'PlotChildren' property, which is an array of object handles.
Putting it all together we get something like the following:
function legappend_HG2(newStrings)
% Quick & dirty fork of legappend specific to MATLAB versions >= R2014b
% Only supports appending strings to the existing legend handle
% Assumes only one legend is in the current figure
% Add multiple strings by passing it a 1D cell array of strings
% Find our legend object
h = findobj(gcf, 'Type', 'legend');
if ~isempty(h)
% Get existing array of legend strings and append our new strings to it
oldstr = h.String;
if ischar(newStrings)
% Input string is a character array, assume it's a single string and
% dump into a cell
newStrings = {newStrings};
end
newstr = [oldstr newStrings];
% Get line object handles
ploth = flipud(get(gca, 'Children'));
% Update legend with line object handles & new string array
h.PlotChildren = ploth;
h.String = newstr;
end
end
Swapping legappend for legappend_HG2 in the above MWE we get the desired result:

Answer 2

Was getting wrong result in R2016b for exkaza's solution. Hence changed the code in a line a little bit.
function legappend_HG2(newStrings)
% Quick & dirty fork of legappend specific to MATLAB versions >= R2016b
% Only supports appending strings to the existing legend handle
% Assumes only one legend is in the current figure
% Add multiple strings by passing it a 1D cell array of strings
% Find our legend object
h = findobj(gcf, 'Type', 'legend');
if ~isempty(h)
% Get existing array of legend strings and append our new strings to it
oldstr = h.String;
%disp(oldstr);
if ischar(newStrings)
% Input string is a character array, assume it's a single string and
% dump into a cell
newStrings = {newStrings};
end
newstr = [oldstr newStrings];
%disp(newstr);
% Get line object handles
ploth = flipud(get(gca, 'Children'));
% Update legend with line object handles & new string array
%h.PlotChildren = ploth;
h.PlotChildren = [h.PlotChildren; ploth];
h.String = newstr;
end
end