When you plot several data series RainPro will work through a standard sequence of colours, shading patterns, markers and so on. These series have been intentionally chosen to be of varying lengths to reduce the chance of any two data-series being plotted in the same combination.

To over-ride the marker series:

chSet 'marker' 2 3 4 112
chSet 'marker' 'Cross,Plus,Diamond'

Note that you can give names or numeric values here. These should be in the range 0-15 to select from the standard markers, or numbers above 15 which are the ids of user-defined markers.

See also ...
chDefineMarker – define markers
Chart Marker Cycle