Improvements/Fixes w.r.t. charts

[liviuchircu]

A few issues that were discovered during OpenSIPS Summit '24 Training in Valencia:

• [Multi-line charts BUG] after creating two statistic groups, both charts seemed to display the same graph. TODO: confirm whether this is a real bug, or other type of issue (perhaps only occurring in certain cases?).
• [Stat monitoring JS behavior] After ticking the checkbox for charting a statistic, the expanded statistic group (e.g. dialog:) collapses! Then, you have to expand it again. You tick the checkbox on the next statistic, causing the group to collapse again! From the user side at least, have to re-expand the group each time feels like a bad UX.
• [Multi-line charts UX] For multi-line charts (statistic groups), there should be a reminder in some shape/form that you need to individually enable each statistic for charting, if you expect it to show on the chart! Otherwise, by only bundling your statistics into a group, the data collection part is yet to be enabled. Intuitively, I think this problem arises because a chart is only created after you click the checkbox, for a given statistic. So naturally, after successfully having created the grouped chart, you don't expect to have to do any other operation: "my multi-line chart shows up now, why does it have no data?!" (typical UX)