Look Ahead setting for graph widgets to show future data

[MrTimbones]

This enhancement adds a "look ahead" setting to graph widgets so that they can show data in the future, for data that's collected from forecasts.

For example: outdoor temperature, with actual vs forecast temperature, set to 24 hours with a 12 hour look ahead. Whenever this graph loads or refreshes, the current time will be in the middle. By chance, zooming out will keep centred on the current time.

Another useful example is to plot the upcoming Agile price:

...and solar power forecasts:

This supersedes PR 143 that also added auto-refresh and has been included into this one

[TrystanLea]

Hello Tim, I think this option should ideally be saved in the saved graph object. Rather than added in as an external parameter. E.g alongside the floatingtime paramater which is probably the best example of such a parameter?

[MrTimbones]

Maybe. I wasn't sure that this would be useful outside of dashboards.

The Graph page doesn't have a use-editable field for floatingtime, it's just set magically based on wherever the user happens to save it. Would it be sensible to automagically set the lookahead if chart is scrolled into the future when saved?