ngToFront functions, by @drewnoakes
 * v1.21: Add 'step' interpolation mode, by @drewnoakes
 * v1.22: Add support for different pixel ratios. Also add optional y limit formatters, by @copacetic
 * v1.23: Fix bug introduced in v1.22 (#44), by @drewnoakes
 * v1.24: Fix bug introduced in v1.23, re-adding parseFloat to y-axis formatter defaults, by @siggy_sf
 * v1.25: Fix bug seen when adding a data point to TimeSeries which is older than the current data, by @Nking92
 *        Draw time labels on top of series