How do you plot markers to specific points?

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DJ Mckenna
DJ Mckenna 2023-5-16
评论： DJ Mckenna 2023-5-16
I have the plot markers added, but they aren't where I need them. They need to be on each line at x= 1,5,9, and 15. How do I achieve this?
  2 个评论
VBBV
VBBV 2023-5-16
编辑：VBBV 2023-5-16
Ran in:
Oneway you can plot markers at specfic points (without using find function) is by creating vector separately and evaluating the equations at those points. Later plot them using hold on
clearvars
x = linspace(1,15,100);
y1 = log(x.^2+4)./(sqrt(2.*x.^2+1));
y2 = sin(x)./log(x.^2)
y2 = 1×100
Inf 3.4370 1.9248 1.3987 1.1153 0.9263 0.7827 0.6638 0.5596 0.4651 0.3772 0.2947 0.2168 0.1431 0.0738 0.0090 -0.0511 -0.1059 -0.1552 -0.1988 -0.2361 -0.2672 -0.2917 -0.3095 -0.3208 -0.3255 -0.3239 -0.3162 -0.3027 -0.2840
% Marker plot
xM = [1 5 9 15];
y3 = log(xM.^2+4)./sqrt(2.*xM.^2+1);
y4 = sin(xM)./log(xM.^2)
y4 = 1×4
Inf -0.2979 0.0938 0.1201
figure
hold on
plot(x,y1,x,y2,'--')
plot(xM,y3,'b+',xM,y4,'r+','MarkerSize',12); grid

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回答（2 个）

Torsten
Torsten 2023-5-16
编辑：Torsten 2023-5-16
The points in the graph you address are x(1), x(5), x(9) and x(15), not x=1, x=5, x=9 and x=15.
Use "find" before plotting to get the respective indices or calculate them by hand.
  1 个评论
Dyuman Joshi
Dyuman Joshi 2023-5-16
编辑：Dyuman Joshi 2023-5-16
Ran in:
OP will not obtain the exact values with the current data, as the values 5 and 9 don't exist in x.
The best idea would be use the colon operator to define x -
x = linspace(1,15,100);
min(abs(x-9))
ans = 0.0606
min(abs(x-5))
ans = 0.0404
%create x via colon operator
x0 = 1:0.01:15; %Ideally x0 = (100:1:1500)/100;
min(abs(x0-9))
ans = 0
min(abs(x0-5))
ans = 0

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Walter Roberson
Walter Roberson 2023-5-16
You have the potential problem that you might not have x coordinates that are exactly 5 or 9 (but exactly 1 and exactly 15 should exist because of the way you linspace()). So if you want markers at x = 5 and x = 9, you have to interpolate positions. The alternative would be to locate the locations in x that are closest to the desired values.
marker_x = [1, 5, 9, 15];
marker_y1 = interp1(x, y1, marker_x);
marker_y2 = interp1(x, y2, marker_x);
h = plot(x, y1, '-', x, y2, '--');
hold on
plot(marker_x, marker_y1, '+', 'Color', h(1).Color);
plot(marker_x, marker_y2, '+', 'Color', h(2).Color);
hold off

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