Make Task History chart more sane

Change the chart type to standard line (no weird steps) so that
all the lines can be tracked clearly, rather than all same-day
changes overlapping each other in one vertical line. Also use
shared tooltip so that information for multiple tasks with the
same number of hours remaining are visible simultaneously.
Sanely handle point overlap in task history chart

Shared tooltip includes everything all the time, which is too
much for many tasks. Non-shared tooltip only shows the data for
one point at a time even when points overlap. So manually format
the tooltip to be 'shared' when hovering over overlapping points.
Check for visible series before adding to tooltip

When a filter is applied on the history charts, the uninteresting
series still exist but are not drawn. If an invisible series
contains an overlapping point with the current hover series, don't
add that point to the tooltip, because it doesn't appear in the
chart.
Revert to step graph with no extra datapoints

The overlapping vertical lines on same-day revisions is a lesser
evil compared to an overload of data point dots for many tasks
on long sprints.
Merge lstarrett's task history chart changes