[RF] Make plotting of histograms work for all orders of magnitude #13316
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lmoneta
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Aug 2, 2023
The RooFit plotting did not work well for variables of small orders of
magnitude, because there was a hardcoded check with an absolute value to
see if two points are considered the same: `x2 - x1 < 1e-20`. If this
condition is true, the point at x2 was not plotted. This meant that some
points that were actually different were skipped.
The fix is to change this condition to a relative one, considering xmin
and xmax. The code to generate sampling hints for histograms was also
moved inside RooCurve, such that it's easier to syncronize the relevant
magic constants.
Here is a demo of a plot that didn't work before but now works:
```
void repro()
{
int nBins = 4;
double scale = 1e-31;
double xmin = 0. * scale;
double xmax = 1. * scale;
double binWidth = (xmax - xmin) / nBins;
// Fill the bin boundaries
std::vector<double> binBoundaries(nBins + 1);
for (int i = 0; i <= nBins; ++i) {
binBoundaries[i] = i * binWidth;
}
// The RooParametricStepFunction needs a TArrayD
TArrayD step_edges{int(binBoundaries.size()), binBoundaries.data()};
RooRealVar Gamma("Gamma", "Gamma", xmin, xmax);
RooArgList step_values;
step_values.addOwned(std::make_unique<RooRealVar>("coef1", "coef1", 0.0 / scale, 0.0, 1.0 / scale));
step_values.addOwned(std::make_unique<RooRealVar>("coef2", "coef2", 0.1 / scale, 0.0, 1.0 / scale));
step_values.addOwned(std::make_unique<RooRealVar>("coef3", "coef3", 0.1 / scale, 0.0, 1.0 / scale));
RooParametricStepFunction prior("prior", "Prior for decay rate", Gamma, step_values, step_edges, nBins);
RooPlot *xframe = Gamma.frame(xmin, xmax);
prior.plotOn(xframe);
auto c1 = new TCanvas();
xframe->Draw();
c1->SaveAs("plot.png");
}
```
This bugfix was inspired by the following forum post:
https://root-forum.cern.ch/t/rooparametricstepfunction/55820
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The RooFit plotting did not work well for variables of small orders of
magnitude, because there was a hardcoded check with an absolute value to
see if two points are considered the same:
x2 - x1 < 1e-20. If thiscondition is true, the point at x2 was not plotted. This meant that some
points that were actually different were skipped.
The fix is to change this condition to a relative one, considering xmin
and xmax. The code to generate sampling hints for histograms was also
moved inside RooCurve, such that it's easier to syncronize the relevant
magic constants.
Here is a demo of a plot that didn't work before but now works:
This bugfix was inspired by the following forum post:
https://root-forum.cern.ch/t/rooparametricstepfunction/55820