why the legend show wrong lines ?

Question

tomer polsky 2019-12-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/497701-why-the-legend-show-wrong-lines

评论： David Goodmanson 2019-12-22

why is the legend of my plot showing the wrong data ? . I am doing plot of this file (DATA_LIM_43_892Khz ) . but for some reason it shows the wrong legend why ?

	ADC_data_lim43=load('Data_LIM_43_892Khz');
    f_RD_not=892.858e3
    real_data_ADC=ADC_data_lim43.real_data_ADC;
    I_real=real_data_ADC*(19.6e-3)/(100*15e-3); %%real current 
    real_data_IN_driver=ADC_data_lim43.real_data_IN_driver; %% real gate signal 
    t=(1/f_RD_not)*(1:length(real_data_ADC));%% time 
    figure(101);
    plot(t,I_real,' red -- d '); %% plot--> of I_real 
    hold on ;
    plot(t,43*(19.6e-3)/(100*15e-3),' black '); %% plot--> of boundery limit  
    hold on ;
    plot(t,100*(19.6e-3)/(100*15e-3)*real_data_IN_driver,' green -- d '); %% plot--> real gate signal   
    title([' I_1=' num2str(43*(19.6e-3)/(100*15e-3)) '[A]']);
    ylabel('I[A]');xlabel('time[Sec]');
    legend ('I_{real}','boundery limit','real gate signal');

as you can see the legend show for some reason wrong data, is this a a bug of matlab ?

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Answer 1

David Goodmanson 2019-12-22

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https://ww2.mathworks.cn/matlabcentral/answers/497701-why-the-legend-show-wrong-lines#answer_407371

编辑：David Goodmanson 2019-12-22

Hi tomer,

try replacing

plot(t,43*(19.6e-3)/(100*15e-3),' black '); %% plot--> of boundery limit

with

plot(t,43*(19.6e-3)/(100*15e-3)*ones(size(t)),' black '); %% plot--> of boundery limit 

and 'hold off' is never a bad idea when you are done adding stuff to the plot.

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tomer polsky 2019-12-22

thank you it works , could you explain why does it works ? or just take it as granted .

David Goodmanson 2019-12-22

HI tomer,

it works because of the following:

x = 1:100;
y = .23;
z = .23*ones(1,100);
figure(1)
plot(x,y)
grid on
figure(2)
plot(x,z)
grid on

The first plot does not give you a trace, but the second one does.

Your original pliot is similar to fig 1, in that it has a scalar y and does not show a black trace. In this situation the intent to produce a flat trace seems clear enough, and Mathworks could have chosen to supply all the constant y values and produce a trace. But for whatever reason they decided not to do that. So to get the trace you have to supply all the values as in fig.2.

In your original plot, the fact that there is a linetype specified, but no trace is produced, seems to confuse the legend command.

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