Exception thrown by memPoolsChart.js on resize.

[hhellyer]

The memory pools chart (Heap Usage) throws exceptions to the JavaScript console when it is resized or goes from visible to invisible:

  | i | @ | d3.v3.min.js:1
-- | -- | -- | --
  | (anonymous) | @ | d3.v3.min.js:3
  | Y | @ | d3.v3.min.js:1
  | Co.each | @ | d3.v3.min.js:3
  | Co.attr | @ | d3.v3.min.js:3
  | resizeMemPoolsChart | @ | memPoolsChart.js:305
  | resize | @ | (index):165
  | (anonymous) | @ | httpThroughPutChart.js:180
  | (anonymous) | @ | d3.v3.min.js:1

It throws four, one for each of the lines it's drawing and I only seem to be able to reproduce this on Chrome.