How should I update the data of a plot in Matlab?

Short answer : always use Set('Xdata',...').

Example code:

function PlotUpdate()   
    x = 0:.1:8;
    y = sin(x);
    h = plot(x,y);
    y = sin(x.^3);    
    set(h,'XData',x,'YData',y);
end

---

Long answer:

There are three relevant measures by which one should choose the best method.

1. Code clarity – How easy it is for someone to read your code?
2. Runtime – How quick each method performs its task?
3. Code portability – How fast can you re-factor your code?

Now, let’s analyze the possible methods.

Method(1) – refreshdata

function PlotUpdate()   
    x = 0:.1:8;
    y = sin(x);
    h = plot(x,y);
    set(h,'YDataSource','y')
    set(h,'XDataSource','x')
    y = sin(x.^3);
    refreshdata(h,'caller');
end

M-lint immediately issues a warning in the line y=sin(x.^3)

The value assigned to variable `y` might be unused

Why does it happen? refreshdata uses eval and m-lint cannot know that you will use y. Someone reading your code, might as well remove this line completely. This happened because you broke the encapsulation principle. refreshdata accesses variables from the caller workspace. Another way to take a look at this, suppose that you pass the handle of the plot to another function. The reader has no clue to why on earth you wrote y = sin(x.^3);, and how is it going to be related to the update of the plot.

Now let’s discuss speed/runtime. By taking a look at refreshdata source code, you will notice two ugly for-loops, that go through all of the graphics handles variables in your space. Here is the first:

% gather up all the objects to refresh
objs = {};
for k = 1:length(h)
  obj = h(k);
  objfields = fields(obj);
  for k2 = 1:length(objfields)
    % search for properties ending in DataSource
    if strncmpi(fliplr(objfields{k2}),'ecruoSataD',10)
      objs = {objs{:},obj, objfields{k2}};
    end
  end
end

Imagine that you have not one plot, but 100 plot and you want to update only the first. This will be very slow, because for each of the plots, you attempt to find the one you need! (I am leaving as an exercise for the reader to figure out what is ecruoSataD, and how it is used.)

Even if you give the relevant plot as an argument, you still have the second loop, that runs eval several times. Not exactly efficient. I will show a time comparison in the end.

Conclusion : Hard to understand, hard to refactor, slow runtime

---

Method (2) – Delete and re-plot

function PlotUpdate()   
    x = 0:.1:8;
    y = sin(x);
    h = plot(x,y);
    set(h,'YDataSource','y')
    set(h,'XDataSource','x')
    y = sin(x.^3);          
    delete(h);
    h = plot(x,y);    
end

This method is quite clear for the reader. You deleted the plot, and drew a new one. However, as we will see from the time comparison in the end, that is the slowest method.

Conclusion : Easy to understand, easy to refactor, very slow runtime

---

Method(3) – set(‘XData’,…,’YData’)

The code is really clear. You want to modify a two properties of your plot, XData and YData. And that is exactly what you do. Also, the code runs really fast, as you can see from the comparison below.

function PlotUpdate()   
    x = 0:.1:8;
    y = sin(x);
    h = plot(x,y);
    y = sin(x.^3);          
    set(h,'XData',x,'YData',y);
end

Since the new graphics engine hg2 (R2014b and up), you can also use property syntax for specifying data if you prefer that notation:

function PlotUpdate()   
    x = 0:.1:8;
    y = sin(x);
    h = plot(x,y);
    y = sin(x.^3);          
    h.XData = x;
    h.YData = y;
end

Conclusion : Easy to understand, easy to refactor, fast runtime

---

Here is the time comparison code

function PlotUpdateTimeCompare()    
    x = 0:.1:8;
    y = sin(x);
    h = plot(x,y);
    set(h,'YDataSource','y')
    set(h,'XDataSource','x')
    y = sin(x.^3);


    tic
    for i=1:100
        refreshdata(h,'caller');
    end
    toc 

    tic
    for i=1:100
        delete(h);
        h = plot(x,y);
    end
    toc     

    tic
    for i=1:100
        set(h,'XData',x,'YData',y);
    end
    toc 

end

And the results:

Elapsed time is 0.075515 seconds.
Elapsed time is 0.179954 seconds.
Elapsed time is 0.002820 seconds.