Add shadows to the histogram chart

I have added a offset and blur filter to add shadows to the histogram
bar charts. With this I have also changed the colors for now.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Use gradient to fill the bar charts

Instead of one color I am now using a gradient to fill the color charts.
I have also removed the shadow effect again, but now it is applied to
the metric meter.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>