plot - Positive & Negitive Log10 Scale Y axis in Matlab

Question

Hi i'm having a problem where I have a dataset which ranges between -10^3 to 10^3

I need to be able to plot this as with a log scale but semilogy cannot plot negative values

Say for example my data is:

x = [-3,-2,-1,0,1,2,3];
y = [-1000,-100,-10,1,10,100,1000];

(or in general y=sign(x).*10.^abs(x);)

How can I plot this in MATLAB with a log scale? If possible It would be great if the log scale ticks could be on the Y-axis too

Answer 1

Use your actual data as labels, but scale the plotted data with log10.

% data
x = -3:0.1:3;
y = sign(x).*10.^abs(x);

% scaling function
scale = @(x) sign(x).*log10(abs(x));

N = 7;    % number of ticks desired

% picking of adequate values for the labels
TickMask = linspace(1,numel(y),N);
YTickLabels = y(TickMask);

% scale labels and plotdata, remove NaN ->inconsistency, do you really want that?
YTick = scale( YTickLabels );
Y = scale(y);

YTick(isnan(YTick)) = 0;
Y(isnan(Y)) = 0;

% plot
plot(x,Y)
set(gca,'YTick',YTick,'YTickLabels',YTickLabels)
grid on

For N = 7:

For N = 11

---

How to find a valid value for N?

The following function (thanks to gnovice) will return all possible values you could choose for N:

n = numel(x);
N = find(rem(n./(1:n), 1) == 0) + 1;

---

about the semilogy-style labels: by adding the following line before the plot:

YTickLabels = cellfun(@(x) ['10^' num2str(x)], num2cell(YTick),'UniformOutput',false)

you could at least achieve something like this: not beautiful and not generic, but a good point to start for you.