CODEWITHSUNDEEP
matlab
William Lindberg
William Lindberg

Reputation: 49

Matlab - how to draw tangent on curve

I have plotted a graph in matlab with:

plot(x,y)

and my graph has different slopes, how do i draw tangents on each slope and calculate the coefficient for the slope?

Upvotes: 3

Views: 20789

Answers (2)

Gunther Struyf
Gunther Struyf

Reputation: 11168

If you don't have an explicit function for the plotted points, you can use finite differences for estimating the derivative. The following is appropriate for points not on the border of the data span:

plot(x,y);
hold all;

% first sort the points, so x is monotonically rising
[x, sortidx] = sort(x);
y = y(sortidx);

% this is the x point for which you want to compute the slope
xslope = (x(1)+x(end))/2;

idx_a = find(x<xslope,1,'last');
idx_b = find(x>xslope,1,'first');
% or even simpler:
idx_b = idx_a+1;
% this assumes min(x)<xslope<max(x)

xa = x(idx_a);
xb = x(idx_b);
slope = (y(idx_b) - y(idx_a))/(xb - xa);

Now drawing that slope, it depends on what you want: just a short line:

yslope = interp1(x,y,xslope);
ya_sloped = yslope + (xa-xslope)*slope;
yb_sloped = yslope + (xb-xslope)*slope;
line([xa;xb],[ya_sloped;yb_sloped]);

or a longer line

yslope = interp1(x,y,xslope);
xa = xa + 4*(xa-xslope);
xb = xb + 4*(xb-xslope);
ya_sloped = yslope + (xa-xslope)*slope;
yb_sloped = yslope + (xb-xslope)*slope;
line([xa;xb],[ya_sloped;yb_sloped]);

I'm pretty sure there're no bugs in this code, but I'll test it out when I have matlab around ;)

Upvotes: 2

blueintegral
blueintegral

Reputation: 1271

You'll have to figure out the slope at whatever points you're interested in using (y2-y1)/(x2-x1) and then use plot() to draw a straight line that has that slope. To draw the line, you need the y intercept, and since you know the coordinates of at least one point on that line (which is the point you want to draw the tangent to), you can solve for b in the equation y=mx+b.

Upvotes: 1

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