Improved/dynamic "aggregation" of data in Cockpit Data Points Graph widget

The default aggregation setting for certain time ranges does not always make sense: for example, plotting values "Daily" for the "Last week" - which only results in 7 values displayed on the chart.

Whilst there are limitations to the number of points that can be drawn on the graph, we would in general like to see better default resolution in the time domain. To achieve this optimally, we suggest that the graph widget should examine the requested time range, and compute an aggregation level that always results in 100s or 1000s or points being displayed.

This would also have the benefit of avoiding the untidy outcome where the data are "truncated" before being displayed on the chart, with an ugly "Truncated data. Change aggregation or select shorter date range." message obscuring the chart.

It is also important that the aggregated data preserves the shape and key features in the data - so that spikes or dramatic changes in the data are still visible. TrendMiner actually does a very good job of this, so we suggest that the same aggregation logic is adopted by Cockpit graphing.