RadCartesianChart still doesn't have a minor ticks support. However, there is a feature request
logged in our feedback portal where you can follow its status and vote for an implementation.
In the meantime you can draw minor ticks and grid lines manually using custom
and grid line
annotations. You can calculate their positions based on the range of the axis and the major tick step. I attached a small project demonstrating a possible implementation. I hope it is useful.
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and share your thoughts.