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.