Asked by Camil Hamel
on 22 Feb 2011

I want to plot a graphic with data that varies over many order (from 10e-4 to 10e-8) with some positive and negative values. Using 'semilog' or 'set xaxis log' can't plot my negative values (witch is normal). When I plot with a normal scale, we don't really see the variability of my datas, we only see the very high values and all the small one are not very visible because there are to close to zero. What would you suggest me to be able to show all my data (even the negative values) and be able to show the variability (like on a log-scale graph). Thank you

Answer by Matt Tearle
on 24 Feb 2011

Accepted Answer

Oookay, I think I have it. If you're going to do this a lot, you may want to make yourself your own function. So, see if this does it for you:

function negsemilogx(x,y)

% Do log10 but keep sign

xlog = sign(x).*log10(abs(x));

% Just to get axis limits

plot(xlog,y,'o')

% Get limits

lims = xlim;

wdth = diff(lims);

% Wrap negative data around to positive side

xlog(xlog<0) = xlog(xlog<0) + wdth;

% Plot

plot(xlog,y,'o')

% Mess with ticks

tck = get(gca,'XTick')';

% Shift those that were wrapped from negative to positive (above) back

% to their original values

tck(tck>lims(2)) = tck(tck>lims(2)) - wdth;

% Convert to string, then remove any midpoint

tcklbl = num2str(tck);

tcklbl(tck==lims(2),:) = ' ';

% Update tick labels

set(gca,'XTickLabel',tcklbl)

Then to try it out:

>> x = -100:100;

>> y = x.^3;

>> negsemilogx(x,y)

Camil Hamel
on 24 Feb 2011

Excellent!

I had to work a little to get exactly what I wanted, but you put me on the right track.

Thanks a lot

Matt Tearle
on 25 Feb 2011

Out of curiosity, what was still missing/wrong?

Camil Hamel
on 25 Feb 2011

Sign in to comment.

Answer by Richard
on 22 Feb 2011

When you choose "log" as the scale for an axis, Matlab either chooses to use log10(x) or -log10(-x) as the transform, depending on whether your data is in the positive or negative domain. As you've found, you can't have both at the same time because both halves map data into the [-inf inf] space.

You can play with your own transforms by transforming the data yourself and at the same time manipulating the XTickLabel property of the axes so that you show untransformed data values. For example, this splices the normal negative and positive transforms, (with the singularity at 0 untransformed):

x = -100:100;

y = x.^3;

xt = x;

xt(xt>0) = log10(1+xt(xt>0));

xt(xt<0) = -log10(1-xt(xt<0));

figure;

ax = axes;

plot(xt,y);

set(ax, 'XTick', [-2 -1 0 1 2], ...

'XTickLabel', {'-100', '-10', '0', '10', '100'});

njj1
on 2 Jun 2016

Sign in to comment.

Answer by Matt Tearle
on 22 Feb 2011

Richard pretty much beat me to it. Here's a slightly more automated way to get the tick marks. However, unfortunately there's no TeX markup for tick labels, so you'll have to choose how you want them to appear -- plain exponent values (2, 3, 4, etc), values written out in full (100, 1000, etc), or some scientific notation (but without TeX markup, eg "10^3" or "1e3")

% Make some fake data

n = 1:20;

y = factorial(n);

y(2:2:end) = -y(2:2:end);

% Bad plot

plot(n,y,'o')

% Transform

ylog = sign(y).*log10(abs(y));

figure

plot(n,ylog,'o')

% Do nothing else to get just exponents. Otherwise:

yt = get(gca,'YTick')';

set(gca,'YTickLabel',num2str(sign(yt).*10.^abs(yt)))

% Or, for scientific notation

set(gca,'YTickLabel',strcat('10^',cellstr(num2str(abs(yt)))))

Sign in to comment.

Answer by Matt Tearle
on 22 Feb 2011

Dumb question, but is there any reason you can't just look at the absolute values? ( abs in MATLAB)

Sign in to comment.

Answer by Camil Hamel
on 22 Feb 2011

Sign in to comment.

Answer by Camil Hamel
on 23 Feb 2011

The figure I have now

x |

x |

x |

x |

| x

| x

| x

| x

| | | | |

-8 -2 0 2 8

meaning

10^-8 10^-2 0 -10^-2 -10^-8

positive data |negative data

__________________________

The figure I would like to have

| x

| x

| x

|x

x|

x |

x |

x |

| | | | |

2 8 -8 -2

meaning

-10^-2 -10^-8 0 10^-8 10^-2

negative data|positive data

Thank you for the answers. I did as Matt said giving the first figure.

I hope you understand the figure I made with my attempt at drawing.

All my data in between -10^-8 , -10^-2 and 10^-2 , 10^-8. So I have to kind of get the value of the zero outside and having my maximum value (10^-8) in the centre of the figure. After that I could change the xticklabel as I want it.

I want to get a figure looking like the second one. Would that be possible and how?

Thank you very much

Sign in to comment.

Answer by Kilbock Lee
on 11 Nov 2013

I think my way is easier using logical operation true false.

Assuming the matrix is P(x,y),

LogP=((P > 0).*log10(P))+(-(P<0).*log10(-P))

and then, plot LogP

First term converts positive values into log scale and Second term does the negative values

Sign in to comment.

Answer by Alexandre HYAFIL
on 13 Apr 2017

Hi everyone,

I have just written a code that replots all the elements in your figure in negative/positive log-scale. It's for the Y axis but could be very simply adapted for the X axis: http://fr.mathworks.com/matlabcentral/fileexchange/62549-yscale-symlog

To use it: x = ... y = ... plot(x,y, ...); yscale_symlog;

Hope it will work for you!

Sign in to comment.

Opportunities for recent engineering grads.

Apply Today
## 0 Comments

Sign in to comment.