[big number] various improvements (#2912)

* [big number] various improvements

* dynamic number of X axis ticks based on width to prevent label overlap
* corrected overflow on the x axis
* improved tooltips (precise arrow and visible data point circle on hover)

* Fixing tooltips in heatmap viz

superset/assets/javascripts/modules/superset.js

@@ -177,7 +177,7 @@ const px = function () {
         $(selector + ' div.alert').remove();
       },
       width() {
-        return token.width();
+        return container.width();
       },
       height() {
         let others = 0;