Add padding to top of assay categories chart
Add padding to top of assay categories chart

This prevents the tallest column in the chart to get its selection triangle cut off. Was doing custom scales to do this, but that causes things like overlapping Y-axis labels. Redmine #4521

ENCODE-DCC/encoded,T2DREAM/t2dream-portal,T2DREAM/t2dream-portal,ENCODE-DCC/encoded,T2DREAM/t2dream-portal,ENCODE-DCC/encoded,ENCODE-DCC/encoded,T2DREAM/t2dream-portal