[script][figure] Change behavior when legends are given for system

Use same legend for dev and eval hist when the number of legends is
equal to the number of systems. It is still possible to change the
legends of individual histograms by passing 2 * #system legends.

bob/measure/script/figure.py

@@ -584,6 +584,10 @@ class Hist(PlotBase):
         self._y_label = 'Probability density'
         self._x_label = 'Scores values'
         self._end_setup_plot = False
+        if self._legends is not None and len(self._legends) == self.n_systems \
+           and not self._hide_dev:
+            self._legends = [x for pair in zip(self._legends,self._legends)
+                             for x in pair]
 
     def compute(self, idx, input_scores, input_names):
         ''' Draw histograms of negative and positive scores.'''