Display Log scale on y axis of graph

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!!

Answer 1

[Rick Rosson]

   >> doc semilogy

[James]

how would i use this with say plot(lamda,intensity,'k-')? and would that just not just give me the log of my ydata

[Rick Rosson]

1. Please read the documentation

2. Please try it. It works.

[James]

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??

Answer 2

[Harry MacDowel]

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);

[James]

I know it might sound slightly absurd but when i was plotting plot(lamda,intensity,'k-') the intensity peaks (y axis) for different temperatures ranged from about 5 to 1e13. The different peaks have to be displayed on the same graph, so what was happening is that the large peaks drowned the small peaks so i cant see them....therefore i need to change the y axis into some kind of log scale so i will be able to see the top of each peak and its value.

i tried using that code but its/im getting stuck on

ytic2str = str2num(ytic2');

this is the error im getting--

??? Error using ==> str2num at 33

Requires string or character array input.

Error in ==> astromodel at 47

ytic2str = str2num(ytic2');

line 32 and 33 of my code is:

32---plot(lambda,b1,'r-');

33---title('Blackbody Radiation Density vs. Wavelenght');

b1 is the intensity as a function of lambda

[Walter Roberson]

num2str() rather than str2num()

[Harry MacDowel]

sorry was writing that piece too fast I didn't notice it.

Yeah Walter got it there. It's num2str not str2num. Sorry.

[James]

ye it compiles nicely but just seams to write over the numbers that are on the graph already.

heres what i have so far, maybe somebody can eventully fix it:

clear all;

h = 6.626e-34; % Planck's Constant = 4.135 x 10^-15 eV s

c = 3e8; % speed of light

T= 1000000; % kelvin

k= 1.38066e-23; % Boltzmann constant in J/K

lambda=0:1e-10:1e-5;

p=8*pi*h*c./(lambda.^5);

b1=p.*1./(exp(h*c./(lambda*k*T)-1));

b2=p.*1./(exp(h*c./(lambda*k*100000)-1));

b3=p*1./(exp(h*c./(lambda*k*10000)-1));

b4=p*1./(exp(h*c./(lambda*k*1000)-1));

b5=p*1./(exp(h*c./(lambda*k*100)-1));

b6=p*1./(exp(h*c./(lambda*k*2.7)-1));

ylimit = [0,35];

ytic = [0 5 10 15 20 25 30 35];

ylimit2 = log(ylimit);

ytic2 = log(ytic);

ytic2str = num2str(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);

plot(lambda,b1,'m-');

title('Blackbody Radiation Density vs. Wavelength');

xlabel('Wavelength [m]')

ylabel('Intensity');

hold on;

plot(lambda,b2,'y-');

plot(lambda,b3,'g-');

plot(lambda,b4,'c-');

plot(lambda,b5,'k-');

plot(lambda,b6,'r-');

legend('1000000K','100000K','10000K','1000K','100K','2.7K');

[Harry MacDowel]

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.