Chart css small fixes

-fix chart tooltip css
-remove redundant display of date (one in the tooltip,
    one at the top of the chart)
-change the legend css (better visual integration)
    - invert background and foreground color
    - remove underline on left tick
    - change tick color to black

Fixes bug #1228031
Change-Id: I45431866cd905209c4dc19f1334beb7cfff54630
Merge "Chart css small fixes"