How to find the intersection of a fitting line with a logarithmic y-axis?

I have a vector A = [10 40 60 80]

and a vecor B = [5 6 7 8]

I need to create a plot where the x-axis is B and the y-axis is A in the logarithmic scale. Then I need to find the intersection of the fitting line of the scatter with the y-axis.

I used semilogy(B,A) but that only gives me the line and I need to find the intersection.

Answer 1

[Kevin Holly]

A= [10 40 60 80];
B = [5 6 7 8];

scatter(B,A,20,'filled')
set(gca,'YScale','log')

% Polyfit
p = polyfit(B,A,2);% 2nd order
f1 = polyval(p,B);
hold on
plot(B,f1,'r--') % poly fit as red dashed line
semilogy(B,A,'g') %log fit as green line

myfit = fittype('a + b*log(x)',...
'dependent',{'y'},'independent',{'x'},...
'coefficients',{'a','b'});
[model stats] = fit(B',A',myfit)

Warning: Start point not provided, choosing random start point.

model = 
     General model:
     model(x) = a + b*log(x)
     Coefficients (with 95% confidence bounds):
       a =      -226.1  (-265.5, -186.7)
       b =       147.4  (126.2, 168.5)

stats = struct with fields:
           sse: 5.9358
       rsquare: 0.9978
           dfe: 2
    adjrsquare: 0.9967
          rmse: 1.7228

plot(B,model(B),'b') % log fit as blue line

I am not quite sure how to calculate the intersection, but found this:

https://www.mathworks.com/matlabcentral/answers/317541-how-can-i-find-the-intersection-from-two-fitting-line