fixed code where d3.format fails for null formats
fixed code where d3.format fails for null formats

ChakriCherukuri/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,SylvainCorlay/bqplot,bloomberg/bqplot,ChakriCherukuri/bqplot,bloomberg/bqplot,dmadeka/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,dmadeka/bqplot