fix plot_input for inputs with a single value

toolkit.py

@@ -1259,7 +1259,10 @@ class ClassificationProject(object):
             centers_sig, hist_sig, _ = self.get_bin_centered_hist(sig, bins=bins, range=plot_range, weights=sig_weights)
             centers_bkg, hist_bkg, _ = self.get_bin_centered_hist(bkg, bins=bins, range=plot_range, weights=bkg_weights)
 
-        width = centers_sig[1]-centers_sig[0]
+        if bins > 1:
+            width = centers_sig[1]-centers_sig[0]
+        else:
+            width = 1.
         ax.bar(centers_bkg, hist_bkg, color="b", alpha=0.5, width=width)
         ax.bar(centers_sig, hist_sig, color="r", alpha=0.5, width=width)