No data for a particular decade

[zaen]

If there is no data for a particular decade then the map will remains same, it shows the last chosen decade.

Check on rock.json dataset:

• Click Genre "Rock"
• Go to decade 1980 then 1970 or from 2000 then directly go to 1970. For the 1970 and 1960, the visualization will remain unchanged.

Probably, we should put the 1970 and 1960 into the json data even tough with an empty value only.