Creating plot with 2 x axes

I have written the following snippet of code for creating a plot with two x axes

figure(),
x1 = linspace(0,100,101);
y1 = sin(x1);
plot(x1,y1)
xlabel('Loss (%)')
ylabel('Energy Efficiency (%)')

x2 = x1*5*40000/100;
ax1 = gca;
ax1_pos = ax1.Position;
ax2 = axes('Position', ax1_pos, 'XAxisLocation', 'top', 'YAxisLocation', 'right');
plot(x2, y1, 'Parent', ax2);

I have configured the 2nd x axis to be on the top, but I get the following, where the two x axes overlap:

I tried the code in this post: https://www.mathworks.com/help/matlab/creating_plots/graph-with-multiple-x-axes-and-y-axes.html?searchHighlight=two%20x-axes

and the code works. But I don't understand what's the problem with my code. Could anybody explain please?

Answer 1

[dpb]

If you'll go through step by step, you'll see the problem doesn't occur until the second plot() call -- you didn't hold the second axes and plot by itself resets things when the 'NextPlot' property is set to default which is 'Replace'

Try something like

...
hAx(1)=gca;
hAx(2)=axes('Position',hAx(1).Position,'XAxisLocation','top','YAxisLocation','right','color','none');
hold(hAx(2),'on')
plot(hAx(2),x2,y1)

and see if joy doesn't ensue...

[Md. Rahman]

Thank you very much. It works for me. Although I don't understand how this works then:

https://www.mathworks.com/help/matlab/creating_plots/graph-with-multiple-x-axes-and-y-axes.html?searchHighlight=two%20x-axes

[dpb]

Because it used line instead of plot to add the data to the axis and line is the low-level routine that just uses the axis as it finds it; plot() is the high-level routine that tries to make things pretty for you.

I probably should have noted the difference rather than just giving the fix; good reminder for future.

The comments on that page should also note the difference explicitly; I'll send TMW support a note on that documentation enhancement.

Answer 2

[Walter Roberson]

Your ax2 is using the default background color, which is white. It is hiding the other axes.

[Md. Rahman]

Thank you very much. Additionally I have to hold axis 2 on, as pointed out by dpb.

Answer 3

[Mckenzie Dice]

I am having a similar probelm but this code does to work for me. I have four lines to plot: three should be on the bottom x axis and the fourth (avri) should be on the top x axis. This code makes the secondary axis but the avri will not plot to it- it still plots to the wrong axis.

for i=1:295
            c=bmus(i);
            subplot(4,5,c)
            hAx(1)=gca;
            plot(av_v(:,c),z_plot, 'r')
            hold on
            plot(av_u(:,c),z_plot,'b')
            hold on
            plot(averages(:,c),z_plot,'g')
            hold on
            xlim([-5,15])
            ylabel('height,m')
            xlabel('u and v (m/s), t (k)','FontSize',10)
            hAx(1)=gca;
            hAx(2)=axes('Position',hAx(1).Position,'XAxisLocation','top','YAxisLocation','right','color','none');
            hold(hAx(2),'on')
            plot(hAx(2),avri(c,:),z_plot,'k')
            xlim([-.05,.15])
            xlabel('Richardson Number','FontSize',10)
            title(c)
            legend('v','u','th','avri')
            fig = gcf;
            fig.Position(3) = fig.Position(3) + 250;
            % add legend
            legend('Position',[0.15 0.95 0.01 0.02])
            legend('boxoff')
        end

[Walter Roberson]

You have hold on but no hold off, so you are plotting 295 times 4 lines in a single plot, but expect that a legend() with 4 items will be enough.

You do not do any drawnow() or pause() or figure(), so you are not animating -- nothing would be drawn on the display until the end of the for i loop.

[Mckenzie Dice]

So where would I put the hold off? just after plotting

plot(hAx(2),avri(c,:),z_plot,'k')

?

[Walter Roberson]

I do not know? Your for runs to 295, but you pull the subplot number on a 4 x 5 grid out of bmus so bmus must have entries no larger than 20. Your current code puts all of the data for each of the distinct bmus values together, an average of 15 different i values for each subplot, and there is no documentation as to what you really want to do.

hAx(2)=axes('Position',hAx(1).Position,'XAxisLocation','top','YAxisLocation','right','color','none');

Note that this creates a new axes() for each of the 295 i values, so you are averaging overlaying 15 different axes on top of each subplot.

[Mckenzie Dice]

So I made the following change

figure(20)
       FigH= figure('Position', get(0, 'Screensize'));
        c=1;
        while c<=20
            subplot(4,5,c)
            plot(av_v(:,c),z_plot, 'r')
            hold on
            plot(av_u(:,c),z_plot,'b')
            hold on
            plot(averages(:,c),z_plot,'g')
            hold on
            xlim([-5,15])
            ylabel('height,m')
            xlabel('u and v (m/s), t (k)','FontSize',8)
            hAx(1)=gca;
            hAx(2)=axes('Position',hAx(1).Position,'XAxisLocation','top','YAxisLocation','right');
            hold(hAx(2),'on')
            plot(avri(c,:),z_plot,'k')
            xlim([-.05,.15])
            xlabel('Richardson Number','FontSize',8)
            title(c)
           % legend('v','u','th','avri')
            fig = gcf;
            fig.Position(3) = fig.Position(3) + 250;
            % add legend
            legend('Position',[0.15 0.95 0.01 0.02])
            legend('boxoff')
            c=c+1;
        end

Now I would like to specify to the code that

plot(av_v(:,c),z_plot, 'r')
            hold on
            plot(av_u(:,c),z_plot,'b')
            hold on
            plot(averages(:,c),z_plot,'g')
            hold on
            xlim([-5,15]) 

should be on one axis (the bottom x axis from -5 to 15)

and plot(avri(c,:),z_plot,'k') should be on the second axis (the top axis from -.05 to .15)