Storing plots as a variable in a for loop

The context to this code is; it is plotting 3 graphs and the data from these graphs need to be combined onto a four graph. So whilst I am processing the data for the first three graphs, I want to plot the relevant graph section for the fourth graph and store it in an array so that I can then call it for the fourth graph and its the legend without having to cycle through the data again.

r_col=[5,8,11]
h_col=[6,9,12]
size=[18,21,25]
xD=[2,6,10]
for z=1:3
    clf
    hold on
    grid on
    r=data(1:size(z),r_col(z))
    h=data(1:size(z),h_col(z))
    v=4*sqrt(h*sin(alpha))
    A(z)=plot(5*v+xD(z)*D,r);
    plot(r,v)
    scatter(r,v)
    name=[int2str(xD(z)),'D Velocity Profile']
    xlabel('Distance from centre line (mm) ');
    ylabel('Velocity of jet (m/s)');
    print(name,'-dpng','-r300');
end    

and then later on when I am doing the legend for the fourth graph, I would like to call it like so:

%applies axis labels
legend([L1,A(1),A(2),A(3),L6,L8,L9],'Core','2D velocities','6D velocities','10D velocities','Nozzle and Origin','Centre Line','Divergence Lines','FontSize',6,'Location','northwest')

So the key issue here is how to store plots so they can be used later?

[Stephen23]

You can obtain the graphics handle to any graphics object:

https://www.mathworks.com/help/matlab/graphics-objects.html

https://www.mathworks.com/help/matlab/creating_plots/how-to-work-with-graphics-objects.html

Note that legend requires axes handles, you just need to obtain the handles to the axes that you create:

axh = axes(...);
...
legend(axh,...)

And of course (because this is MATLAB) you can create arrays of handles too, so you can use indexing as you show.

You should also read the documentation carefully to know what objects you are dealing with, for example plot returns an array of line objects (which are children of an axes object).

[Joshua Martin]

Unfortunately, due to my lack of experience. I don't understand where my code/logic is wrong.

By my intuition, I am creating a handle for each plot and storing them in the array A. However, when I try to call these I found that these handles have been deleted. Also, I want to apply these graph sections to the axis that the rest of the graphs are, and so I don't understand why I would need to supply the legend with the axis handle and not just the plot handle.

If you could provide some more information as I unfortunately cannot understand the links provided.

[Tommy]

"clf deletes from the current figure all graphics objects whose handles are not hidden (i.e., their HandleVisibility property is set to on)."

When you call clf in your loop, the most recent graphics object stored in A (as well as every other graphics object in the current figure) is deleted.

>> A = plot(1:10)

A = 

  Line with properties:

              Color: [0 0.4470 0.7410]
          LineStyle: '-'
          LineWidth: 0.5000
             Marker: 'none'
         MarkerSize: 6
    MarkerFaceColor: 'none'
              XData: [1 2 3 4 5 6 7 8 9 10]
              YData: [1 2 3 4 5 6 7 8 9 10]
              ZData: [1×0 double]

  Show all properties

>> clf
>> A

A = 

  handle to deleted Line

[Joshua Martin]

So if my understanding of your answer is correct, I should just set

A(z).Visible='off'

or alternatively

set(A(z),'Visible',' off ')

and then this would be ignored by clf and so I can continue to store it.

However, I still get the same deleted output as you show above.

Further to this,

findobj('type','line')

still outputs the data for the plot despite it in theory not being able to due to the plot not being visible.

I don't see why the plot would still be visible.

Any workaround for this?

[Stephen23]

The problem is clf: if you delete all the existing objects on each iteration, then they don't exist after the loop.

I would probably do this using one axes, where for three iterations you:

• hide any existing objects (do not delete them)
• plot some new data (i.e. add new line objects, allocating their handles to an array)
• export the image

and then after the loop use the array of line objects to make them all visible again (for the fourth image). This would be much more robust and clearer if you used explicit handles for all objects, probably preallocating the line handle array using gobjects.

"I don't understand why I would need to supply the legend with the axis handle and not just the plot handle."

You do not have to pass the axes, passing an array of line objects is also okay. Whatever works for you.

Note that MATLAB does not have a "plot handle" because there is no "plot" type object. When you call plot it returns the handle to a line object, as the documentation explains.

[Joshua Martin]

Thank you, the code now works although I ended up using a normal array rather than a graphical array as the first object in a graphical array tells us that it is a graphical array which was messing with my code. I'm sure theres a simple fix such as indexing (z+1).

For anyone that uses this, here's an extract of code that worked for me.

%sets columns to read r, h, the size of each column, ratio of x/D, the
%multiplication factor (fac) for Q and the inital Q value.
r_col=[5,8,11];
h_col=[6,9,12];
size=[15,21,25];
xD=[2,6,10];
fac=[4,5,5];
Q=[pi*(D*10^-3)^2/fac(1)*max(v_cl)];

%clears previous plots and opens an empty array to store plot handles
clf
plots=[];
for z=1:3
    hold on
    grid on
    %takes r, h and v values for each distance, calculates the flow rate
    %and adds to array,
    r=data(1:size(z),r_col(z))
    h=data(1:size(z),h_col(z))
    v=4*sqrt(h*sin(alpha));
    Q=[Q sum(v.*pi.*abs(r)*fac(z)*10^-6)]
    %plots the relevant section of data needed for the plan view and sets
    %to not visible so not to interfere until needed later
    A(z)=plot(5*v+xD(z)*D,r);
    set(A(z),'Visible','off')
    %plots the distance graphs whilst putting into an array, these are then
    %printed and set to not visible (clf would delete the graph made in A)
    plots=[plots plot(r,v) scatter(r,v)];
    name=[int2str(xD(z)),'D Velocity Profile']
    xlabel('Distance from centre line (mm) ');
    ylabel('Velocity of jet (m/s)');
    print(name,'-dpng','-r300');
    set(plots,'Visible','off');
end    

%code for fourth graph goes here

set(A,'Visible','on');
legend([A(1),A(2),A(3)],'Nozzle and Origin','Centre Line','Divergence Lines','FontSize',6,'Location','northwest')

[Tommy]

Note that the Visible property is not the same as the HandleVisibility property. clf and findobj care about the latter, not the former:

>> a = axes;
>> clf
>> a

a = 

  handle to deleted Axes

versus

>> a = axes;
>> a.HandleVisibility = 'off';
>> clf
>> a

a = 

  Axes with properties:

             XLim: [0 1]
             YLim: [0 1]
           XScale: 'linear'
           YScale: 'linear'
    GridLineStyle: '-'
         Position: [0.1300 0.1100 0.7750 0.8150]
            Units: 'normalized'

  Show all properties

and

>> A = plot(1:10, 'HandleVisibility', 'off');
>> findobj('type','line')

ans = 

  0×0 empty GraphicsPlaceholder array.

>> A.HandleVisibility = 'on';
>> findobj('type','line')

ans = 

  Line with properties:

              Color: [0 0.4470 0.7410]
          LineStyle: '-'
          LineWidth: 0.5000
             Marker: 'none'
         MarkerSize: 6
    MarkerFaceColor: 'none'
              XData: [1 2 3 4 5 6 7 8 9 10]
              YData: [1 2 3 4 5 6 7 8 9 10]
              ZData: [1×0 double]

  Show all properties

[Joshua Martin]

Perfect, thank you. That clears up the confusion I was having and allows me to delete the plots after use rather than accumulate hidden plots.