Returns the chart's convertible series: visible (optionally restricted to
indices) and not internal. Highstock injects internal helper series (the
navigator preview, marked via isInternal / the highcharts-navigator-series
class) that mirror real data and must never become their own layers.
Returns the chart's convertible series: visible (optionally restricted to
indices) and not internal. Highstock injects internal helper series (the navigator preview, marked viaisInternal/ thehighcharts-navigator-seriesclass) that mirror real data and must never become their own layers.