Add margins to histogram SVGs (#1085)

* Add a left margin to histogram SVGs

Fixes https://github.com/chanzuckerberg/cellxgene/issues/1042

1. Added "margins" to the SVG as in https://bl.ocks.org/mbostock/3019563
2. Tailored the brushable area to be just where the data is and locked
   puppeteerUtils to use that in e2e testing.

* Dial in margins to avoid y-axis clipping

client/__tests__/e2e/data.js

@@ -41,7 +41,7 @@ export const datasets = {
         {
           metadata: "n_genes",
           "coordinates-as-percent": { x1: 0.25, y1: 0.5, x2: 0.55, y2: 0.5 },
-          count: "1537"
+          count: "1552"
         }
       ]
     },