the killer feature for me in a charting library is the ability to have multiple Y-axis scales. Say I want to have three time-series, so I am plotting efficiency in %, power in megawatts and flow in m3/s on the Y-axis and time on the x-axis, three Y-axis scales are required.

Usually I end up with multiple charts or some SCADA program has made it so the Y-axis scale changes when you select a series if there is only 1 chart.