generic_axis.js: no more than 18 labels on the x-axis of a bar plot

Better spaced in the general case, and should prevent clashes until a 1280x resolution.
See #2697 and e4670ae1.

browser/js/generic_axis.js

@@ -35,7 +35,7 @@ function GenericAxis (reverse, can_undefined) {
     this.value_mapping = {};
     this.can_undefined = true;
     this.NB_STEPS_IN_AXIS = 6; // Number (max) of labels per numerical axis
-    this.NB_STEPS_BAR = 30; // Number (max) of labels per numerical axis in histograms
+    this.NB_STEPS_BAR = 18; // Number (max) of labels per numerical axis in histograms
     if(typeof can_undefined !== "undefined")
         this.can_undefined = can_undefined;
     this.reverse = false;