Mark a point on graph

Question

Leonor Vieira Dias 2021-2-1

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/733263-mark-a-point-on-graph

评论： Star Strider 2021-2-1

Hello,

I have made a graph with the following code. My goal was to know whey the y-axis is equal to 0.331. This means, the time it takes for the concentration of the lake to be 0.331. I also wanted to mark that point in my graph. Is that possible?

Thank you in advance

First, let's define our variables:

r = 10^5 ;
V = 10^10 ;
c_i = 100 ;
t = 0:10^7 ;

Then, we enter our model for the concentration in the lake :

c_L = c_i.*(1-exp((-r.*t)./V));

Finally, we plot the evolution of the concentration in the lake:

plot(t,c_L);
semilogx(c_L);
xlabel("Time t (hours)");
ylabel("Concentration of pollutant in the lake cL (kg/m^3)");
title("Concentration vs time");

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Answer 1

Star Strider 2021-2-1

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/733263-mark-a-point-on-graph#answer_612078

在 MATLAB Online 中打开

Try this:

r = 10^5 ;
V = 10^10 ;
c_i = 100 ;
t = logspace(-2, 7, 20);
c_L = c_i.*(1-exp((-r.*t)./V));
Q1 = nnz(diff(c_L)==0)
c_crit = 0.331;
t_crit = interp1(c_L, t, c_crit);
figure
plot(t,c_L);
hold on
plot(t_crit, c_crit, 'sr', 'MarkerFaceColor','r')
hold off
set(gca, 'XScale','log', 'YScale','log')
% semilogx(c_L);
xlabel("Time t (hours)");
ylabel("Concentration of pollutant in the lake cL (kg/m^3)");
title("Concentration vs time");
text(t_crit, c_crit, sprintf('  \\leftarrow t = %5.1f, Concentration = %6.3f', t_crit, c_crit), 'HorizontalAlignment','left', 'VerticalAlignment','middle')