Primary (left) y-axis interpolation mode: ScaleType::Linear (default) or ScaleType::Log. ScaleType::DateTime is not meaningful for a value axis. Log scale clamps non-positive data to a small positive floor (see Scale::log_nice) — suited to network traffic, latency, or financial data spanning multiple orders of magnitude. CartesianOptions::begin_at_zero is ignored when this is Log (a log domain can't include zero). The secondary (right) y-axis is Linear-only in v1.