Show aggregation interval legend

[vkaverin]

For some time in past, or zoomed out plots, aggregation interval really is not the same as it's specified in the view settings, it can be larger due to historical data or specified time period.

So for better understanding of the picture it would be nice to see some kind of aggregation legend for the plot.

Here you can see that most of the plot has smoother values, which is actually aggregated by 5 minutes, than the right side, and the legend will tell the reason of the noise, which is aggregated by 1 minute.