rflot: flotSeries: When grouping using a factor in the data.table, allow
series to be colored according to numeric value.

According to

https://github.com/flot/flot/blob/master/API.md#customizing-the-data-series

the color argument can be an actual color or a numeric specifying which
color to use from the internal array.  This can be beneficial when
trying to build out a facet from a single data.table in so you can
preserve the series coloring across different charts.