I have a problem with semilogx command

Hello,

I want to have the x axis as a log scale however it doesn't want to work. This function basically plots cubic splines in order to generate a p-v diagram, however when i run the code it only gets the splines that are shaded onwards, so the first half of the graph is missing.

Any help?

Thank you

function pv_plot(V,c1,c2,c3, c4, V_input_1, V_input_2)

for i = 2:72
    x = V(i-1):((V(i+1)- V(i))/100):V(i);
    y = (c1(i).*((V(i+1)-x).^3))+(c2(i).*((x-V(i)).^3))+(c3(i).*(V(i+1)-x))+(c4(i).*(x-V(i)));

    semilogx(x,y)
    
if V(i-1)>V_input_1 && V(i)<V_input_2
    H = area (x,y, 'EdgeColor', 'r');
    hold on;
    set(H(1), 'FaceColor', 'r');
end

end

hold off

end

[Walter Roberson]

Your plot has broken lines that hint you have infinity or nan values in your inputs.

[dpb]

Let's see enough to reproduce the actual data. What was the input to the function or attach x,y as .mat file.

semilogx won't handle negative values if something goes wrong for x...that we can't see.

Walter's comment is apropos, too, fer shure...

[Nadine Nassar]

X is an array of 73 values of volume. (0.00316 to 219.9)

Y is a cubic spline i created

The function basically requires two values of volume then outputs the corresponding pressure (through cubic spline interpolation) and the boundary work (area between v1 and v2) and the bulk modulus (differentiation).

The graph should plot the whole pv curve but im not sure why it doesnt show the whole graph

[Walter Roberson]

Copies of V,c1,c2,c3, c4, V_input_1, V_input_2 would allow us to test the code to track down the problem.