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matlabplotmatlab-figurelegend

Making a legend when some of the data sets could be empty

For my project I have six sets of data to put on a scatter plot, like so:

plot(ax, ay, '.r', bx, by, '.b', cx, cy, '.m', dx, dy, '.c', ex, ey, '.y', fx, fy, '.k');

Sometimes these sets of data will be empty, so bx and by might have nothing in them, therefore getting skipped over.

Is there any way to build a legend that will match the right label to the right color piece of data? In other words, data in the [cx, cy] would always match the label 'c' on the legend next to a magenta dot, even when there is no 'b'. My current legend is as follows: 

legend('a', 'b', 'c', 'd', 'e', 'f', -1);

Thanks!

Solution

  • You can get the results you want if you first replace any sets containing empty data with NaN values. For example:

    x = [];
y = [];

if isempty(x) || isempty(y)
  x = nan;
  y = nan;
end

plot(1:10, rand(1,10), 'r', x, y, 'g', 1:10, rand(1,10), 'b');
legend('r', 'g', 'b');

    enter image description here

    Leaving x and y as empty would give a warning when creating the legend and result in an incorrect legend. When passing empty vectors ([]) to the plot command, it simply doesn't create a line object for that data. When passing a NaN value, it creates but doesn't render a line object, so it still exists and can have a legend entry generated for it.