How can I plot negative value with log scale?

I want plot some data which varies over many order (from 1e6 to 1e-4) with some positive and negative values in log scale(like the attached picture), but the matlab function -- 'loglog' can only plot either positive or negative data in one drawing. Is there any function can plot both the positive and negative data? just like the 'symlog' scale in matplotlib. Or any other alternative way?

[dpb]

Attach your desired plot(*)...not sure what negative values in log space are???

(*) Yeah, the interface is funky, you have to "Attach" the figure also after you've selected it from the file dialog...

[Qing-quan Zhi]

Thanks! The figure has been attached.

[dpb]

That's a pretty funky scale as well as the interface... :)

The range from 10^-2 to 0 is same as that from 10^-2 to 10^0 (two decades) although log(0) --> -Inf.

What one could do, I suppose, is have two axes, one with normal and other reversed y-axes directions and then fix up the middle y tick label with the introduction of the fake zero and and the minus signs that can't ever be in log space. The lower half axes would have to use the abs(y) in the semilogy call, of course.

[Qing-quan Zhi]

The scale is very useful when displaying the data over many order. An frequently-used method is transforming data by the formular t = sign(x)*log(abs(x)), then plot the (t,y) instead of (x,y), and modify the yTickLabel manually.

[dpb]

Yeah, but...what do you do about the issue that the last decade covers range of infinite number of decades on the same scale as the previous that is actually only one? Truncate data to eliminate it or what???

Answer 1

[Robert]

Rather than use the transform you mentioned in your comment,

 t = sign(x)*log(abs(x))

you could use

 t = sign(x)*log(1+abs(x)/10^C)

which would preserve the continuity of your plot across zero and allows you to tune the visibility into values near zero. As long as you are careful to label the axes appropriately I don't think it is fair to call this incorrect or deceptive.

You can edit the Tick marks and labels and even modify the MinorTicks of the [X,Y,Z]Ruler object as occurs when you use set(gca,'XScale','log') or the like.

Inspired by your question, I made a function called symlog to do just this and put it on the File Exchange here.

An example plot:

x = linspace(-50,50,1e4+1);
y1 = x;
y2 = sin(x);
y3 = x - sin(x);

plot(x,y1,x,y2,x,y3)
symlog(gca,'xy',-1.7)

[Qing-quan Zhi]

Yeah, the bi-symmetric logarithmic axes is very useful way to display some data, although it is not really accurate in math. And you give a perfect solution! Thank you!

[Jungmin kim]

I'm really grateful for your work. Your creations have significantly saved me time.

Answer 2

[the cyclist]

This strikes me as an incorrect and potentially deceptive way to plot these data.

Instead, I think I would use the subplot function, putting the positive data in the upper subplot and the negative data into the lower subplot. Then, there would be a "chasm" between the two, which makes sense because they really are separated in log space.

Granted that by default the two plots would take up some real estate on the page, but you can adjust properties to minimize the impact of that.

Answer 3

[Rajen Biswa]

I have these data: Ebbyno=[ -5 -3 -1 1 3 5 7 9 11 13 15}; already in dB. BER1= [ 0.3993 0.3769 0.3445 0.3015 0.2091 0 0 0 0 0 0 0 0]; how can i use the loglog plot to plot this data in matlab.