Heatmap crashes with d3 errors when the x and y axis label arrays only contain a single string

[wobenshain]

http://codepen.io/wobenshain/pen/XKYmGA

This appears to be an unintended consequence of the fix for Issue #605 which causes v0 to be set to a string instead of V0In or 0.

Relevant commit: b913a5b

[etpinard]

fixed via #783