Require GD::Graph 1.47 for the hide_overlapping_values layout option

Otherwise labels may not be visible at all.
@ChartColors config option

Primarily for access from other parts of the charting code, such as
table rendering, but also useful for anyone who wants to adjust the
defaults to something more pleasing.
Color the chart table column headings to match the image

Now no legend is needed and a quick visual association is possible.