Don't ignore the zoom slider

[stuartlangridge]

The slider under a graph alters the time period which the graph covers. However, if the graph gets redrawn (by choosing Weekly or Monthly, or making the graph full-size) then it ignores the current zoom slider setting and just draws it at its default time period, but leaves the slider where it is.

This should abide by the current setting of the slider and draw the graph with that zoom.

[stuartlangridge]

OK. We currently explicitly and deliberately ignore the zoom slider: in particular, we remember separate settings for the zoom slider for each type of graph. (So if you set the slider to something for the Monthly graph, then switch to Weekly and set the slider to something different, then switch back to Monthly, we should reset the slider to the thing you picked for Monthly. See rememberedSliderValues in https://github.com/MeasureOSS/Measure/blob/master/assets/js/chartjs-plugin-adjustable.js#L29.)

However, this feat of memory is not saved anywhere, such as across a page reload. We presumably did this because it seemed like a good idea at the time, although it's possible that now what we think is a sensible thing to do has changed and the slider should carry on across different graph types (and fullscreen graphs): that is, the new proposal is that moving the slider sets a time period to show in the graph (say, from October 2019 to today) and that time period should be persisted across all graphs. But I'm loath to change something where we made a deliberate decision to do it another way without checking why we did that, so @jeremy-lq do you think it is correct to change this so that the time period is what the slider sets, and that time period is what is maintained across different examples of the same graph (so setting the slider to Oct19-today makes that the period shown on both the weekly and the monthly graphs in this widget)?

[jeremy-lq]

I think setting the slider to Oct19-today should definitely persist from graph -> fullscreen graph. I don't think the current behavior was deliberate, so also keeping it when switching from Monthly to Weekly seems to more intuitive behavior.

[stuartlangridge]

PR for testing.