How can i plot a line from an already plotted line?

I'm doing a code for brake study and, I've been trying to add a proportioning valve to my code. It essencialy creates a line in the optimum cuve graph. It is similar to this green curve:

Does anyone know how to adjust the starting point of the green line, so it starts from any poiny at the black line? Here is the method I used for this graph:

p2(32) = plot(ar_VC(find(ax_VC==ak):end),af_VC(find(ax_VC==ak):end),'-','Color', 1/255*[70,250,120],'LineWidth',3);

[Voss]

How do you plot the black line?

[Daniel Bengtson]

To clarify, are you wanting to translate the green line in its entirety to a new starting point or are you wanting to trim the green line such that it starts at its intersection with the black line?

[Rodrigo]

i want to translate the green line to a starting point where it stars anywhere on the black line.

And the black line was plotted like the code below, it´s a bunch of books which resulted on the black line.

p2(30) = plot(ar_4x2,af_4x2,'-','Color',1/255*[0,0,0],'LineWidth',3);

[Sam Chak]

@Rodrigo, If you have a starting point (pivot) fixed on the black line, then the green line projected from the pivot, looks like rotated about the pivot relative to the black line.

Answer 1

[Daniel Bengtson]

% for a given value of X that lands on the defined range of the black line,
% define the corresponding Y value
X = 5;

blackX = ar_4x2;
blackY = af_4x2;

greenX = ar_VC(find(ax_VC==ak):end);
greenY = af_VC(find(ax_VC==ak):end);

% create first order fit of black line to allow for arbitrary X input
p = polyfit(blackX,blackY,1);
Y = polyval(p,X);

% shift green X and Y based on the difference between green(1) and black(1)
% then account for the translation along the black line
greenX = greenX - (greenX(1) - blackX(1)) + X;
greenY = greenY - (greenY(1) - blackY(1)) + Y;

figure;
p2(30) = plot(blackX,blackY,'-','Color',1/255*[0,0,0],'LineWidth',3);
hold on
p2(32) = plot(greenX,greenY,'-','Color', 1/255*[70,250,120],'LineWidth',3);