Problems with lin/log-Scale


im having problems with the scaling of my graphs. The gPad->SetLogy()
command switches the axis labeling but does not alter the shape of the
functions (exp stays exp, where it should be linear) that i have on my pad - am I missing something?

An example code to reproduce the problem would look like this:
TCanvas *canv = new TCanvas( “gl_gLifeTime” , “Life Time”, 1024, 768 );
TF1 *f1 = new TF1( “f1” , “[0] + [1]exp(-1x/[2])” , 0, 200 );
Double_t fitpar[3];
fitpar[0] = 73.0f;
fitpar[1] = 130.0f;
fitpar[2] = 20;

Additionally im attaching two pictures, one in lin-scale the other one
in log-scale to demonstrate the output i get on my machine (arch-linux,
root installed via Arch User Repository).

Thanks a lot

fitpar[0] = 0.0;

You are right, that solves the representation problem - but my function has a constant part, so I would like to have a way without changing my original function (in the sample code its of course just a simple function - in my real data its a messy fitfunction with a TGraphErrors-Object). I do not understand why ROOT should have a problem with an offset in my functions…
Additional hints … log(a + b) = ? … log(a + b * c) = ? … and if you now put a = 0.0 …

Now I see the point :wink: Tank you.

So, in order to get the desired result with the following graph the only way is to subtract the offset from all the points?? Or is there a more elaborate approach to it when handling TGraphs?

If you subtract an “offset”, you will possibly get some points which have non-positive “y” values -> then you will not be able to draw them in log-y mode.
Try to draw … “ExperimentalData / FittedFunction” … which should be a nice “flat” graph/histogram around 1, or maybe … “ExperimentalData - FittedFunction” … which should also be a nice “flat” graph/histogram around 0. No need to use log-y mode in these cases.