[Feature request] Variables: Allow empty value

Hello, We're using Grafana with OpenTSDB, running on CentOS (not sure about the Grafana version, sorry; I can check if necessary). I love templating and I'm using it a lot but I'm pretty frustrated that template variables do not seem to allow empty values. I think there's a number of valid use cases for this (at least when working with OpenTSDB, maybe with other datasources as well but I wouldn't know about that): - When using template variables for tag values: groupBy will be disabled in case the corresponding tag value is empty → it would be extremely comfortable to be able to specify an empty value and thus very easily disable groupBy and see the aggregation instead. - When using template variables for down sample intervals: again, it would be very comfortable to be able to leave the variable empty so that down sampling defaults to "auto" mode. I hope you'll see as much interest in this feature as I do and that it can fit in the existing philosophy of the product! ;-) Many thanks, Laurent