Update charts every minute and fix `yMax` computation

Fix #13 (closed)

I'm now refecthing the historical values every minute. This is done by subscribing to the historical attributes themselves instead of their corresponding "live" attributes (i.e. current_history instead of current). Whenever we receive a new historical value (i.e. every minute), we force a refetch of all the historical values for that attribute. This is simpler and more robust than to save the new historical values as they come in.

I'm also fixing a couple of overflow issue with the charts, as well as the computation of yMax which was not dealing with NaN correctly. The lifetime chart this morning before/after the fix: