That's because when you evaluate
x(set>0)
y(set>0)
you just get vectors which are interpreted as single lines
x(set>0)
ans =
0.3 0.4 0.7 0.8
y(set>0)
ans =
0.29552 0.38942 0.64422 0.71736
If I understand you correctly, you want the value in set(i) to determine the line width of line segment xy(i) to xy(i+1). So the length of 'set' should be 1 less than the length of x or y.
A simple way to achieve that would be to plot each segment in a loop. There's surely a non-loop method available that might sacrifice readability and simplicity.
figure
axh = axes;
hold(axh, 'on')
for i = 1:length(x)-1 %-1 because you're plotting by line segment
ph = plot(axh, x(i:i+1), y(i:i+1), 'color', 'b');
if set(i)==1
ph.LineWidth = 5;
end
end
