Octave one line two plot differ from multiple line plot

Question

I'm fitting some data, but when I plot the data and fit in one-line-plot (see left figure) the drawn graph is correct

plot(x, y, '.b;data;', [0.05 2], phi(1)+phi(2)*[0.05 2], '--r;fit;', [0.05 2]);

But when I used two separated plot (see right figure) the graph differs from mention above

hold on;

plot(x, y, '.b;data;', [0.05 2]);
plot(phi(1)+phi(2)*[0.05 2], '--r;fit;', [0.05 2]);

hold off;
grid on;

Data

[x, y]
ans =
    0.050000     3571.000000
    0.100000     6567.000000
    0.200000    12760.000000
    0.300000    20512.000000
    0.400000    25480.000000
    0.500000    32088.000000
    1.000000    63223.000000
    2.000000   128690.000000

Calculate Linear Regression

A = [N, sum(x); sum(x), sum(x.*x)];
b = [sum(y); sum(x.*y)];
phi = inv(A)*b;

Is there any way to solve this?

Answer 1

When the x-axis values are not specified then 1:numel(y) are considered to be the x-axis values.

In your code:

%Your first graph:
plot(x, y, '.b;data;', [0.05 2], phi(1)+phi(2)*[0.05 2], '--r;fit;', [0.05 2]);
                                                                     %^^^^^^^

%Your second graph:

plot(x, y, '.b;data;', [0.05 2]);
                      %^^^^^^^^^

plot(phi(1)+phi(2)*[0.05 2], '--r;fit;', [0.05 2]);
    %!!!!!!!!!!!!!!!!!!!!!!             %^^^^^^^^^

The parts highlighted with ^ in above plot commands have x=[1 2] and y=[0.05 2] and the part highlighted with !has x=[1 2] and y=phi(1)+phi(2)*[0.05 2] . You can see those lines being drawn if you zoom at that area.

So your first plot command should be:

plot(x, y, '.b;data;', [0.05 2], phi(1)+phi(2)*[0.05 2], '--r;fit;');

and it should be split like this:

plot(x, y, '.b;data;'); 
hold on;
plot([0.05 2], phi(1)+phi(2)*[0.05 2], '--r;fit;');