In Matlab, how to draw lines from the curve to specific xaxis position?

问题

I have a spectral data (1000 variables on xaxis, and peak intensities as y) and a list of peaks of interest at various specific x locations (a matrix called Peak) which I obtained from a function I made. Here, I would like to draw a line from the maximum value of each peaks to the xaxis - or, eventually, place a vertical arrow above each peaks but I read it is quite troublesome, so just a vertical line is welcome. However, using the following code, I get "Error using line Value must be a vector of numeric type". Any thoughts?

X = spectra;
[Peak,intensity]=PeakDetection(X);
nrow = length(Peak);
Peak2=Peak;  % to put inside the real xaxis value 
plot(xaxis,X);
hold on
for i = 1 : nbrow
        Peak2(:,i) = round(xaxis(:,i));  % to get the real xaxis value and round it
        xline = Peak2(:,i);
        line('XData',xline,'YData',X,'Color','red','LineWidth',2);
end
hold off

回答1:

Simple annotation:

Here is a simple way to annotate the peaks:

plot(x,y,x_peak,y_peak+0.1,'v','MarkerFaceColor','r');

where x and y is your data, and x_peak and y_peak is the coordinates of the peaks you want to annotate. The add of 0.1 is just for a better placing of the annotation and should be calibrated for your data.
For example (with some arbitrary data):

x = 1:1000;
y = sin(0.01*x).*cos(0.05*x);
[y_peak,x_peak] = PeakDetection(y); % this is just a sketch based on your code...
plot(x,y,x_peak,y_peak+0.1,'v','MarkerFaceColor','r');

the result:

---

Line annotation:

This is just a little bit more complicated because we need 4 values for each line. Again, assuming x_peak and y_peak as before:

plot(x,y);
hold on
ax = gca;
ymin = ax.YLim(1);
plot([x_peak;x_peak],[ymin*ones(1,numel(y_peak));y_peak],'r')
% you could write instead:
% line([x_peak;x_peak],[ymin*ones(1,numel(y_peak));y_peak],'Color','r')
% but I prefer the PLOT function.
hold off

and the result:

---

Arrow annotation:

If you really want those arrows, then you need to first convert the peak location to the normalized figure units. Here how to do that:

plot(x,y);
ylim([-1.5 1.5]) % only for a better look of the arrows
peaks = [x_peak.' y_peak.'];
ax = gca;
% This prat converts the axis unites to the figure normalized unites
% AX is a handle to the figure
% PEAKS is a n-by-2 matrix, where the first column is the x values and the
% second is the y values
pos = ax.Position;
% NORMPEAKS is a matrix in the same size of PEAKS, but with all the values
% converted to normalized units
normpx = pos(3)*((peaks(:,1)-ax.XLim(1))./range(ax.XLim))+ pos(1);
normpy = pos(4)*((peaks(:,2)-ax.YLim(1))./range(ax.YLim))+ pos(2);
normpeaks = [normpx normpy];
for k = 1:size(normpeaks,1)
    annotation('arrow',[normpeaks(k,1) normpeaks(k,1)],...
        [normpeaks(k,2)+0.1 normpeaks(k,2)],...
        'Color','red','LineWidth',2)
end

and the result:

来源：https://stackoverflow.com/questions/41647411/in-matlab-how-to-draw-lines-from-the-curve-to-specific-xaxis-position