Add a 45-degree line in a log-glog plot

Question

Md Mia 2023-1-6

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1889322-add-a-45-degree-line-in-a-log-glog-plot

评论： Md Mia 2023-1-7

采纳的回答： Voss

Hi

I have a scatter plot as below:

scatter(log(X(:,1)),log(X(:,2)))

I want to add a 45-degree line here. I tried using refline(1,0) and got followint fig. Does it okay or refline can't be used in log-log plot. Please help.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Voss 2023-1-6

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1889322-add-a-45-degree-line-in-a-log-glog-plot#answer_1142692

Well, that's not a log-log plot; both axes are linear.

The line does indeed have slope 1. It may not be apparent that the slope is 1 because the x- and y-scales are different. (Notice that the distance from 19 to 20 on the x-axis is about twice as many screen pixels as the distance from 19 to 20 on the y-axis.)

4 个评论
显示 2更早的评论隐藏 2更早的评论

John D'Errico 2023-1-7

在 MATLAB Online 中打开

@Md Mia - your confusion stems from the fact that IF the axis scaling is not the same for x and y, then a 45 degree line will not look like it is 45 degrees.

Let me give you an example.

x = [0 1];
y = [0 1];

Do you agree that the curve represented by those points MUST be a slope of 1? So the line would be perfectly 45 degrees? Now I'll plot those points TWICE, but I'll change the axes.

plot(x,y,'-')

xlim([-1 2])

ylim([0,1])

plot(x,y,'-')

xlim([0,1])

ylim([0,1])

axis equal

axis tight

The second plot was created to have the same spacing on the x and y axes. (Though, strangely, the y axis can sometimes have a different set of tick marks than x. Sigh.)

But do you agree that the first plot does not look remotely as if it has a 45 degree slope? In fact, it does have a perfectly 45 degree slope, but only when viewed properly. Screw around with the axis scaling, and nothing will look right. The data was the same on both plots.

Now, refline(1,0) DOES create a 45 degree line. But if you use it to plot a line on a set of axes where the units are differently scaled, then you get something that will look wrong, no matter what. That does not make it wrong. It is just your perception of wrong-ness that will be the problem.

Md Mia 2023-1-7

Thanks for the clarifications. I understood now. Changed my scaling and look better although previously also it was okay. Thank you John D'Errico for your excellent explanations.

请先登录，再进行评论。