Apply different shading to date histogram bars that are only partially included in a date range

[yawitz]

An idea that emerged from issue #664 (Pick a date range other then 1st date of the month , bar charts aren't highlighted):

Users can select a date range that starts and/or ends in the middle of a month. Currently, the date histogram bars for such partial "edge months" are not highlighted. It would be nice to draw these bars differently, e.g. using a dashed line or pattern fill or some other visual treatment.