Apply stretchMargin only to distribution bar (#1124)

* When the label size is too short, the constant for calculating
margin_size does not apply. Also nvd3 auto-adjust font-size of axis
labels.
Temporary solution here: Setting a fixed font-size on nvd3 axis labels
and a minimum threshold for label size.

* Only stretch margin for dist_bar

caravel/assets/visualizations/nvd3_vis.js

@@ -312,8 +312,12 @@ function nvd3Vis(slice) {
         }
 
         if (fd.bottom_margin === 'auto') {
-          const stretchMargin = calculateStretchMargins(payload);
-          chart.margin({ bottom: stretchMargin });
+          if (vizType === 'dist_bar') {
+            const stretchMargin = calculateStretchMargins(payload);
+            chart.margin({ bottom: stretchMargin });
+          } else {
+            chart.margin({ bottom: 50 });
+          }
         } else {
           chart.margin({ bottom: fd.bottom_margin });
         }