Some reasons for this request:

• the color yellow is very hard to read on white background
  Example: Plot 1, data series bbb
• The same data series in different plots should have the same colours for better recognition
  Example: Data series ccc is green in Plot 1 but blue in Plot 2
• The colours used change with the number of data series
  Example: 2nd data series is yellow in Plot 1 but light green in Plot 2
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