LXW_CHART_TRENDLINE_TYPE_POLY

Undocumented in source but is binding to C. You might be able to learn more by searching the web for its name.
extern (C)
enum LXW_CHART_TRENDLINE_TYPE_POLY = lxw_chart_trendline_type.LXW_CHART_TRENDLINE_TYPE_POLY;

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