Colors should be stable in plots

[Steve Magoun]

Each time you change a plot (for example by turning on/off one of the bug statuses), the colors in the plot change. They shouldn't - the plots would be easier to read.

To reproduce:
1) Collect data from a project
2) Click the 'Plot' tab
3 Plot the data; note the colors used for the lines in the plot
4) Un-check one of the 'Series' (New, Incomplete, etc)

Expected results:
The colors used in the plot don't change

Actual results:
The colors used in the plot change each time you adjust the series in the plot