"polyxpoly" doesn't intersect lines correctly

I have some grain size data plotted in a semilogx plot and would like to find out the cumulative percentages for different abundance levels. To do this I have been using "polyxpoly" but the intesection points don't align where I'd expect

In terms of the code used to produce the intersects:

intersect_x = 0.0005:0.0005:0.255;

intersect_y(1:510) = 0.84;

for x = 1:height(wolman.s1_us)

date = datestr(wolman.s1_us.Time(x),'dd-mm-yyyy');

semilogx(grain_size,wolman.s1_us{x,:},"DisplayName",date,"LineWidth",lW);

hold on

[xi(x),yi(x)] = polyxpoly(grain_size,wolman.s1_us{x,:},intersect_x,intersect_y);

scatter(xi,yi,"DisplayName",date)

end

line3 = yline(0.84,'-',yLineLabels(3),"DisplayName",yLineLabels{3});

Any help on this would be greatly appeciated!

Answer 1

[Ayush]

Hey Josh,

I understand that you are facing an issue with finding the cumulative percentages for different abundance levels based on grain size data plotted in a “semilogx” plot. You have been using the "polyxpoly" function to find the intersection points, but they do not align as expected.

For reproducing the exact issue at my end, code is required.

Meanwhile, you can convert the data to linear scale before using “polyxpoly” and then convert the resulting intersection points back to the logarithmic scale as the “polyxpoly” function operates on linear coordinates and may not give accurate intersection points when used with logarithmic scales. Here’s an example of how you can modify your code:

intersect_x = 0.0005:0.0005:0.255;
intersect_y = 0.84 * ones(size(intersect_x));

for x = 1:height(wolman.s1_us)
    date = datestr(wolman.s1_us.Time(x), 'dd-mm-yyyy');
    semilogx(grain_size, wolman.s1_us{x, :}, 'DisplayName', date, 'LineWidth', lW);
    hold on
    
    % Convert to linear scale for intersection calculation
    linear_grain_size = 10.^grain_size;
    linear_data = 10.^wolman.s1_us{x, :};
    
    % Calculate intersection points in linear scale
    [linear_xi, linear_yi] = polyxpoly(linear_grain_size, linear_data, intersect_x, intersect_y);
    
    % Convert intersection points back to logarithmic scale
    xi = log10(linear_xi);
    yi = log10(linear_yi);
    
    scatter(xi, yi, 'DisplayName', date);
end

line3 = yline(0.84, '-', yLineLabels(3), 'DisplayName', yLineLabels{3});

For more information on “semilogx” plot and “polyxpoly”, refer to MathWorks documentation link below:

1. https://in.mathworks.com/help/matlab/ref/semilogx.html
2. https://in.mathworks.com/help/map/ref/polyxpoly.html

Hope this information helps!

Regards,

Ayush Goyal