Add named data series and multi-axis charts

Multi-dimensional measurements now support named data series with independent units, enabling richer time-series visualizations.

• Added name field to data series, allowing each axis to carry a label (e.g. "Frequency", "Voltage")
• Moved units from measurement level to individual data series for per-axis unit control
• Added multi-axis Y chart support with independent scales when series have different units
• Added interactive series toggle in chart legend to show/hide individual data series