Heatmap redraw function is called when the component is not displayed #635

See the heatmap pattern overview page on patternfly.org:
http://www.patternfly.org/pattern-library/data-visualization/heat-map/#/overview

In the console you will see a number of errors resulting from the redraw method being invoked while the parent of the component is display: none

Some error handling should be added to the component redraw method to handle cases where elements are not displayed, and certain attributes are not able to be looked up.