const size_t bins = gsl_histogram_bins(hist);
const double x_pos = (ch->data_right - ch->data_left) * bar / (double) bins ;
const double width = (ch->data_right - ch->data_left) / (double) bins ;
const size_t bins = gsl_histogram_bins(hist);
const double x_pos = (ch->data_right - ch->data_left) * bar / (double) bins ;
const double width = (ch->data_right - ch->data_left) / (double) bins ;