Fetch new sensor data when saving `sensors_to_show`

[Flix6x]

The chart specs do update, but no new sensor data is fetched, so for new data to show up, users need to do either a page refresh or select a new date period. I think the new data fetch should happen automatically, after first the chart specs update (as already happens now). That is, the chart would refresh twice: once fast (with the new chart specs) and once less fast (with the new chart data).