# Display Log scale on y axis of graph

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James on 2 Oct 2011
Hi, if anybody could help me with this i would be really greatful, im trying to plot the blackbody radiation for a number of temperatures from 3 kelvin - 1million kelvin
However it is very hard to see the smaller plots due to the weights of the high temperatures, therefore i need the y axis to increase logarithmically WITHOUT getting the log of the data. so how can i change the axis????
thanks in advance, if you can answer this within a day you are my new hero!!

Rick Rosson on 2 Oct 2011
>> doc semilogy
James on 2 Oct 2011
used semilogy(lambda,b1,'r-') and variations of it a load of times now, fine it gives me a logarithmic axis but its takking a log of my data which is not what i want, iv only been using matlab for about a week, maybe im missing something??

Harry MacDowel on 2 Oct 2011
A log scale but not the log of your data, that is exactly what semilogy is doing.
In fact I think you are not stating your problem very clearly either. What do you really want?
If you have a log axis, but then your data is not logarithmic, do you mean that you don't want to transform the data through a calculation process of your own/the Matlab or, you want to plot the 'original' data under a log scale? That would be quite absurd since your data would be jam-packed into a bundle of dots 'collapsing' onto each other.
If you really want to do this, you can achieve it by transforming the axis. Do something like,
ylimit = [0,35];
ytic = [0 5 10 15 20 25 30 35];
ylimit2 = log(ylimit);
ytic2 = log(ytic);
ytic2str = str2num(ytic2');
ax1 = gca;
ax2 = axes('Position',get(ax1,'Position'),'Color','none');
set(ax2,'ylim',ylimit,'ytick',get(ax1,'ytick'),'xtick',get(ax1,'xtick'),'xticklabel',[],'yticklabel',ytic2str);
Harry MacDowel on 8 Oct 2011
yes that code is replacing the labels on the y-axis without altering anything in regard to the original plot or the data itself. That's why I am asking you in the first place, what do you want?
Please do realize that currently the ytic in the codes I given hasn't been altered by you either.
If you want to view the data on a log scale way above the limit of your current y data, please change the limit yourself and thus the ensuing codes.

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