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matlabplotcdf

CDF plot doesn't start from zero

I've tried to plot cdfcurves of my data by calling cdfplot(x) function. Here is part of my code:

    figure; 
    for i=1:length(num_curves)
        h = cdfplot(10*log10(plot_vec1(:,i)));
        set(h,'Linewidth',2,'Color',cc(i,:),'Linestyle','-');
        hold on
        %set(color, cc(i,:));
    end

Unfortunately the result I get is very confusing. enter image description here

Yellow and purple curves are starting from nonzero value, but CDF curve should always start from zero! Can anyone give me some suggestions? I've tried to plot yellow curve by itself, but it is still 'biased'. Thank you for help!

Solution

  • The problem is that your data plot_vec1 contains zeros. Those zeros are transformed to -inf when you transform to dB with 10*log10(). Then cdfplot sees that the minimum finite value (about -67 in your yellow curve) has many samples (with value -inf) below it. That's why it gives a non-zero accumated probability there.

    For example, compare these two figures

    1. Normal case, all values are finite:

      cdfplot([1 2 3 4 5 6])

      enter image description here

    2. Some values are -inf:

      cdfplot([-inf -inf 3 4 5 6])

      enter image description here

      Here the minimum finite value, which is 3, has a 0.33 cumulative probability shown on the vertical axis. That's because 2 of the 6 samples are - inf and are thus below 3:

    Possible workarounds: remove those zeros before applying the logarithm, or replace the resulting -inf by a very small value in dB, such as -100.