X-axis interpolation mode: ScaleType::Linear (default, evenly-spaced categories) or ScaleType::DateTime (continuous domain positioned by real value, via CartesianChart::set_x_values). Only honored by chart types that override CartesianChart::supports_continuous_x_axis (LineChart, AreaChart) — ignored (falls back to category mode) on BarChart/ComboChart, whose fixed-width bar-per-category layout doesn't have a well-defined meaning for irregularly-spaced continuous positions.