Axis drop threshold was too high (minimum 30 entries). When the filter selection result resulted in a small sample, all of the resulting data was completely thrown away. This patch changes the default minimum of non-zero entities per axis to 1.
Rewrite of heatmap axis threshold dropping logic
Drop columns and rows if at least 4 records do not exist
Revert to threshold = 1 for dropna
Merge pull request #20 from UPDDI/Issue-18

Merge Fixes for Issue #18